Also in 1913, the Irish Citizen Army was founded by the trade unionists and socialists James Larkin and James Connolly following a series of violent incidents between trade unionists and the Dublin police in the Dublin lock-out.

On 25 November 1913, the Irish Volunteers were formed by Eoin MacNeill in response to the paramilitary Ulster Volunteer Force that had been founded earlier in the year to fight against Home Rule.

In June 1914, Nationalist leader John Redmond forced the Volunteers to give his nominees a majority on the ruling committee.

In September 1914, Redmond encouraged the Volunteers to enlist in the British Army, a faction led by Eoin MacNeill broke with the Redmondites, who became known as the National Volunteers, rather than fight for Britain in the war.

The British Parliament passed the Third Home Rule Act on 18 September 1914 with an amending Bill for the partition of Ireland introduced by Ulster Unionist MPs, but the Act's implementation was immediately postponed by the Suspensory Act 1914 due to the outbreak of the First World War in the previous month. The majority of nationalists followed their IPP leaders and John Redmond's call to support Britain and the Allied war effort in Irish regiments of the New British Army, the intention being to ensure the commencement of Home Rule after the war.

The plan for revolt was realised in the Easter Rising of 1916, in which the Volunteers launched an insurrection whose aim was to end British rule.

In April 1916, Irish republicans launched the Easter Rising against British rule and proclaimed an Irish Republic. Although it was crushed after a week of fighting, the Easter Rising and the British response led to greater popular support for Irish independence.

This further alienated Irish nationalists and produced mass demonstrations during the Conscription Crisis of 1918. In the 1918 general election Irish voters showed their disapproval of British policy by giving Sinn Féin 70% (73 seats out of 105) of Irish seats, 25 of these uncontested.

In April 1918, the British Cabinet, in the face of the crisis caused by the German Spring Offensive, attempted with a dual policy to simultaneously link the enactment of conscription into Ireland with the implementation of Home Rule, as outlined in the report of the Irish Convention of 8 April 1918.

In early July 1918, Volunteers ambushed two RIC men who had been stationed to stop a feis being held on the road between Ballingeary and Ballyvourney in the first armed attack on the RIC since the Easter Rising – one was shot in the neck, the other beaten, and police carbines and ammunition were seized.

In the December 1918 election, the republican party Sinn Féin won a landslide victory in Ireland.

On 21 January 1919 they formed a breakaway government (Dáil Éireann) and declared Irish independence.

Sinn Féin won 91% of the seats outside of Ulster on 46.9% of votes cast, but was in a minority in Ulster, where unionists were in a majority. Sinn Féin pledged not to sit in the UK Parliament at Westminster, but rather to set up an Irish Parliament. This parliament, known as the First Dáil, and its ministry, called the Aireacht, consisting only of Sinn Féin members, met at the Mansion House on 21 January 1919.

While it was not clear in the beginning of 1919 that the Dáil ever intended to gain independence by military means, and war was not explicitly threatened in Sinn Féin's 1918 manifesto, an incident occurred on 21 January 1919, the same day as the First Dáil convened. The Soloheadbeg Ambush, in County Tipperary, was led by Seán Treacy, Séumas Robinson, Seán Hogan and Dan Breen acting on their own initiative.

A policy of ostracism of RIC men was announced by the Dáil on 11 April 1919.

The Oath of Allegiance to the Irish Republic which the Dáil had instated on 20 August 1919.

The British forces, in trying to re-assert their control over the country, often resorted to arbitrary reprisals against republican activists and the civilian population. An unofficial government policy of reprisals began in September 1919 in Fermoy, County Cork, when 200 British soldiers looted and burned the main businesses of the town, after one of their number – a soldier of the King's Shropshire Light Infantry who was the first British Army death in the campaign – had been killed in an armed raid by the local IRA on a church parade the day before (7 September).

The treaty allowed Northern Ireland, which had been created by the Government of Ireland Act 1920.

By mid-1920, the Irish Republic was a reality in the lives of many people, enforcing its own law, maintaining its own armed forces and collecting its own taxes.

In mid-1920, Republicans won control of most county councils, and British authority collapsed in most of the south and west, forcing the British government to introduce emergency powers.

By 1920, the IRP had a presence in 21 of Ireland's 32 counties.

The British increased the use of force; reluctant to deploy the regular British Army into the country in greater numbers, they set up two paramilitary police units to aid the RIC. The Black and Tans were seven thousand strong, mainly ex-British soldiers demobilized after World War I. Deployed to Ireland in March 1920, most came from English and Scottish cities. While officially they were part of the RIC, in reality, they were a paramilitary force.

In March 1920, Tomás Mac Curtain, the Sinn Féin Lord Mayor of Cork, was shot dead in front of his wife at his home, by men with blackened faces who were seen returning to the local police barracks. The jury at the inquest into his death returned a verdict of wilful murder against David Lloyd George (the British Prime Minister) and District Inspector Swanzy, among others. Swanzy was later tracked down and killed in Lisburn, County Antrim. This pattern of killings and reprisals escalated in the second half of 1920 and in 1921.

In early April 1920, 400 abandoned RIC barracks were burned to the ground to prevent them being used again, along with almost one hundred income tax offices. The RIC withdrew from much of the countryside, leaving it in the hands of the IRA.

The Irish Republican Police (IRP) was founded between April and June 1920, under the authority of Dáil Éireann and the former IRA Chief of Staff Cathal Brugha to replace the RIC and to enforce the ruling of the Dáil Courts, set up under the Irish Republic.

In May 1920, Dublin dockers refused to handle any war matériel and were soon joined by the Irish Transport and General Workers' Union, who banned railway drivers from carrying members of the British forces. Blackleg train drivers were brought over from England, after drivers refused to carry British troops. The strike badly hampered British troop movements until December 1920, when it was called off.

In June–July 1920, assizes failed all across the south and west of Ireland; trials by jury could not be held because jurors would not attend. The collapse of the court system demoralised the RIC and many police resigned or retired.

The first cycle of attacks and reprisals broke out in the summer of 1920. On 19 June a week of inter-sectarian rioting and sniping started in Derry, resulting in 18 deaths.

In July 1920, another quasi-military police body, the Auxiliaries, consisting of 2,215 former British army officers, arrived in Ireland. The Auxiliary Division had a reputation just as bad as the Tans for their mistreatment of the civilian population but tended to be more effective and more willing to take on the IRA. The policy of reprisals, which involved public denunciation or denial and private approval, was famously satirised by Lord Hugh Cecil when he said: "It seems to be agreed that there is no such thing as reprisals but they are having a good effect".

On 17 July 1920, a British Colonel Gerald Smyth was assassinated by the IRA in the County Club in Cork city in response to a speech that was made to police officers of Listowel who had refused orders to move into the more urban areas, in which he stated "you may make mistakes occasionally, and innocent persons may be shot, but that cannot be helped.

On 21 July 1920, partly in response to the killing of Smyth and partly because of competition over jobs due to the high unemployment rate, loyalists marched on the Harland and Wolff shipyards in Belfast and forced over 7,000 Catholic and left-wing Protestant workers from their jobs. Sectarian rioting broke out in response in Belfast and Derry, resulting in about 40 deaths and many Catholics and Protestants being expelled from their homes.

The British Liberal journal, The Nation, wrote in August 1920 that "the central fact of the present situation in Ireland is that the Irish Republic exists".

On 9 August 1920, the British Parliament passed the Restoration of Order in Ireland Act. It replaced the trial by jury by courts-martial by regulation for those areas where IRA activity was prevalent.

On 22 August 1920, RIC Detective Swanzy was shot dead by Cork IRA men while leaving a church in Lisburn, County Antrim. Swanzy had been blamed by an inquest jury for the killing of Cork Mayor Tomás Mac Curtain.

Between 1 November 1920 and 7 June 1921 twenty-four men were executed by the British.

Violence escalated steadily from that summer and sharply after November 1920 until July 1921. (It was in this period that a mutiny broke out among the Connaught Rangers, stationed in India. Two were killed whilst trying to storm an armory and one was later executed).

About 300 people had been killed by late 1920, but the conflict escalated in November. On Bloody Sunday in Dublin, 21 November 1920, fourteen British intelligence operatives were assassinated in the morning; then in the afternoon the RIC opened fire on a crowd at a Gaelic football match, killing fourteen civilians and wounding 65. A week later, seventeen Auxiliaries were killed by the IRA in the Kilmichael Ambush in County Cork.

Then, on 21 November 1920, there was a day of dramatic bloodshed in Dublin. In the early morning, Collins' Squad attempted to wipe out the leading British intelligence operatives in the capital. The Squad shot 19 people, killing 14 and wounding 5. These consisted of British Army officers, police officers and civilians. The dead included members of the Cairo Gang and a courts-martial officer, and were killed at different places around Dublin.

On 28 November 1920, only a week after Bloody Sunday in Dublin, the west Cork unit of the IRA, under Tom Barry, ambushed a patrol of Auxiliaries at Kilmichael in County Cork, killing all but one of the 18-man patrol.

In the Government of Ireland Act 1920 (enacted in December 1920), the British government attempted to solve the conflict by creating two Home Rule parliaments in Ireland: Northern Ireland and Southern Ireland. While Dáil Éireann ignored this, deeming the Irish Republic to be already in existence, Unionists in the north-east accepted it and prepared to form their own government.

On 10 December 1920, martial law was proclaimed in Counties Cork, Kerry, Limerick and Tipperary in Munster; in January 1921 martial law was extended to the rest of Munster in Counties Clare and Waterford, as well as Counties Kilkenny and Wexford in Leinster.

On 11 December, the centre of Cork City was burnt out by the Black and Tans, who then shot at firefighters trying to tackle the blaze, in reprisal for an IRA ambush in the city on 11 December 1920 which killed one Auxiliary and wounded eleven.

In January 1921, two years after the war had started, the Dáil debated "whether it was feasible to accept formally a state of war that was being thrust on them, or not", and decided not to declare war.

During the following eight months until the Truce of July 1921, there was a spiraling of the death toll in the conflict, with 1,000 people including the RIC police, army, IRA volunteers and civilians, being killed in the months between January and July 1921 alone.

On 1 February, the first execution under martial law of an IRA man took place. Cornelius Murphy of Millstreet, Cork was shot in Cork city.

on 11 March, Dáil Éireann President Éamon de Valera called for acceptance of a "state of war with England". The Dail voted unanimously to empower him to declare war whenever he saw fit, but he did not formally do so.

On 19 March 1921, Tom Barry's 100-strong West Cork IRA unit fought an action against 1,200 British troops – the Crossbarry Ambush. Barry's men narrowly avoided being trapped by converging British columns and inflicted between ten and thirty killed on the British side.

On 21 March, the Kerry IRA attacked a train at the Headford junction near Killarney.

The failure of the British efforts to put down the guerrillas was illustrated by the events of "Black Whitsun" on 13–15 May 1921.

A general election for the Parliament of Southern Ireland was held on 13 May. Sinn Féin won 124 of the new parliament's 128 seats unopposed, but its elected members refused to take their seats.

The biggest single loss for the IRA, however, came in Dublin. On 25 May 1921, several hundred IRA men from the Dublin Brigade occupied and burned the Custom House (the center of local government in Ireland) in Dublin city center. Symbolically, this was intended to show that British rule in Ireland was untenable. However, from a military point of view, it was a heavy defeat in which five IRA men were killed and over eighty captured.

On 6 June 1921, the British made their first conciliatory gesture, calling off the policy of house burnings as reprisals. On the other side, IRA leaders and in particular Michael Collins, felt that the IRA as it was then organized could not continue indefinitely. It had been hard-pressed by the deployment of more regular British soldiers to Ireland and by the lack of arms and ammunition.

On 24 June 1921, the British Coalition Government's Cabinet decided to propose talks with the leader of Sinn Féin. Coalition Liberals and Unionists agreed that an offer to negotiate would strengthen the Government's position if Sinn Féin refused.

Seizing the momentum, Lloyd George wrote to Éamon de Valera as "the chosen leader of the great majority in Southern Ireland" on 24 June, suggesting a conference.

By July 1921 there were 50,000 British troops based in Ireland; by contrast, there were 14,000 soldiers in metropolitan Britain.

On 10 July 1921, the IRA ambushed British forces in Raglan street in Belfast.

The war of independence in Ireland ended with a truce on 11 July 1921.

While the fighting in the south was largely ended by the Truce on 11 July 1921, in the north killings continued and actually escalated until the summer of 1922.

Most IRA officers on the ground interpreted the Truce merely as a temporary respite and continued recruiting and training volunteers. Nor did attacks on the RIC or British Army cease altogether. Between December 1921 and February of the next year, there were 80 recorded attacks by the IRA on the soon to be disbanded RIC, leaving 12 dead.

The Peace talks led to the negotiation of the Anglo-Irish Treaty (6 December 1921).

Anglo-Irish Treaty was then ratified in triplicate: by Dáil Éireann on 7 January 1922.

Winston Churchill arranged a meeting between Collins and James Craig on 21 January 1922 and the southern boycott of Belfast goods was lifted but then re-imposed after several weeks.

On 18 February 1922, Ernie O'Malley's IRA unit raided the RIC barracks at Clonmel, taking 40 policemen prisoner and seizing over 600 weapons and thousands of rounds of ammunition.

In April 1922, in the Dunmanway killings, an IRA party in Cork killed 10 local suspected Protestant informers in retaliation for the shooting of one of their men. Those killed were named in captured British files as informers before the Truce signed the previous July.

In May and June 1922, Collins launched a guerrilla IRA offensive against Northern Ireland. By this time, the IRA was split over the Anglo-Irish Treaty, but both pro and anti-treaty units were involved in the operation. Some of the arms sent by the British to arm the new Irish Army were in fact given to IRA units and their weapons sent to the North.

On 22 May, after the assassination of West Belfast Unionist MP William Twaddell, 350 IRA men were arrested in Belfast, crippling its organisation there.

The largest single clash came in June, when British troops used artillery to dislodge an IRA unit from the village of Pettigo, killing seven, wounding six and taking four prisoners. This was the last major confrontation between the IRA and British forces in the period 1919–1922.

The cycle of sectarian atrocities against civilians however continued into June 1922. May saw 75 people killed in Belfast and another 30 died there in June. Several thousand Catholics fled the violence and sought refuge in Glasgow and Dublin.

On 17 June, in revenge for the killing of two Catholics by the B-Specials, Frank Aiken's IRA unit shot ten Protestant civilians, killing six in and around Altnaveigh, south Armagh. Three Special Constables were also killed in the shootings.

The outbreak of the civil war in the South ended the violence in the North, as the war demoralized the IRA in the northeast and distracted the attention of the rest of the organization from the question of partition. After Collins' death in August 1922, the new Irish Free State quietly ended Collins' policy of covert armed action in Northern Ireland.

The violence in the north fizzled out by late 1922, the last reported killing of the conflict in what was now Northern Ireland took place on 5 October.

On 6 December 1922, following the coming into legal existence of the Irish Free State, W. T. Cosgrave became President of the Executive Council, the first internationally recognized head of an independent Irish government.

The Free State if it wished, which it duly did on 8 December 1922 under the procedures laid down.

Irish Civil War lasted until mid-1923 and cost the lives of many of the leaders of the independence movement, notably the head of the Provisional Government Michael Collins, ex-minister Cathal Brugha, and anti-treaty republicans Harry Boland, Rory O'Connor, Liam Mellows, Liam Lynch and many others: total casualties have never been determined but were perhaps higher than those in the earlier fighting against the British. President Arthur Griffith also died of a cerebral hemorrhage during the conflict.

By February 1923, under the 1922 Special Powers Act the British were detaining 263 men on Argenta, which was moored in Belfast Lough. This was supplemented with internment at other land based sites such as Larne workhouse, Belfast Prison and Derry Gaol. Together, both the ship and the workhouse alone held 542 men without trial at the highest internment population level during June 1923.