The Irish Republican Army (IRA) is a name used by various paramilitary organisations in Ireland throughout the 20th and 21st centuries. Organisations by this name have been dedicated to irredentism through Irish republicanism, the belief that all of Ireland should be an independent republic free from British rule. They are designated terrorist organisations in Ireland and in the United Kingdom.The original Irish Republican Army (19191922), often now referred to as the "old IRA", was raised in 1917 from members of the Irish Volunteers and the Irish Citizen Army, later reinforced by Irishmen, formerly in the British Army in World War I, who returned to Ireland to fight against Britain in the Irish War of Independence. In Irish law, this IRA was the army of the revolutionary Irish Republic as declared by its parliament, Dil ireann, in 1919.

In the century that followed, the original IRA was reorganised, changed and split on multiple occasions, to such a degree that many subsequent paramilitary organisations have been known by that title most notably the Provisional Irish Republican Army, which was a key participant during the Troubles. The contemporary IRA organisations each claim the sole right to the name, each insisting they are the original IRA's only legitimate descendant.



The Irish War of Independence (Irish: Cogadh na Saoirse) or Anglo-Irish War was a guerrilla war fought in Ireland from 1919 to 1921 between the Irish Republican Army (IRA, the army of the Irish Republic) and British forces: the British Army, along with the quasi-military Royal Irish Constabulary (RIC) and its paramilitary forces the Auxiliaries and Ulster Special Constabulary (USC). It was part of the Irish revolutionary period.

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 Rising and the British response led to greater popular support for Irish independence. In the December 1918 election, 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. That day, two RIC officers were killed in the Soloheadbeg ambush by IRA volunteers acting on their own initiative. The conflict developed gradually. For most of 1919, IRA activity involved capturing weaponry and freeing republican prisoners, while the Dáil set about building a state. In September, the British government outlawed the Dáil and Sinn Féin and the conflict intensified. The IRA began ambushing RIC and British Army patrols, attacking their barracks and forcing isolated barracks to be abandoned. The British government bolstered the RIC with recruits from Britain—the Black and Tans and Auxiliaries—who became notorious for ill-discipline and reprisal attacks on civilians, some of which were authorised by the British government. Thus the conflict is sometimes called the "Black and Tan War". The conflict also involved civil disobedience, notably the refusal of Irish railwaymen to transport British forces or military supplies.

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. 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; then the RIC fired on the crowd at a Gaelic football match, killing fourteen civilians and wounding sixty-five. A week later, the IRA killed seventeen Auxiliaries in the Kilmichael Ambush in County Cork. In December, the British authorities declared martial law in much of southern Ireland, and the centre of Cork city was burnt out by British forces in reprisal for an ambush. Violence continued to escalate over the next seven months, when 1,000 people were killed and 4,500 republicans were interned. Much of the fighting took place in Munster (particularly County Cork), Dublin and Belfast, which together saw over 75 percent of the conflict deaths.The conflict in north-east Ulster had a sectarian aspect. While the Catholic minority there mostly backed Irish independence, the Protestant majority were mostly unionist/loyalist. A mainly-Protestant special constabulary was formed, and loyalist paramilitaries were active. They attacked Catholics in reprisal for IRA actions, and in Belfast a sectarian conflict raged in which almost 500 were killed, most of them Catholics. In May 1921, Ireland was partitioned under British law by the Government of Ireland Act, which created Northern Ireland.

A ceasefire began on 11 July 1921. The post-ceasefire talks led to the signing of the Anglo-Irish Treaty on 6 December 1921. This ended British rule in most of Ireland and, after a ten-month transitional period overseen by a provisional government, the Irish Free State was created as a self-governing Dominion on 6 December 1922. Northern Ireland remained within the United Kingdom. After the ceasefire, violence in Belfast and fighting in border areas of Northern Ireland continued, and the IRA launched a failed Northern offensive in May 1922. In June 1922, disagreement among republicans over the Anglo-Irish Treaty led to the eleven-month Irish Civil War. The Irish Free State awarded 62,868 medals for service during the War of Independence, of which 15,224 were issued to IRA fighters of the flying columns.