The Irish Republican Army (IRA) of 1922–1969, an anti-Treaty sub-group of the original Irish Republican Army (1919–1922), fought against the British-backed Irish Free State in the Irish Civil War, and its successors up to 1969, when the IRA split again into the Provisional IRA and Official IRA. The original Irish Republican Army fought a guerrilla war against British rule in Ireland in the Irish War of Independence between 1919 and 1921. Following the signing of the Anglo-Irish Treaty on 6 December 1921, the IRA in the 26 counties that were to become the Irish Free State split between supporters and opponents of the Treaty. The anti-Treatyites, sometimes referred to by Free State forces as "Irregulars",A continued to use the name "Irish Republican Army" (IRA) or in Irish Óglaigh na hÉireann, as did the organisation in Northern Ireland which originally supported the pro-Treaty side. Óglaigh na hÉireann was also adopted as the name of the pro-Treaty National Army, and remains the official legal title of the Irish Defence Forces.

The Irish Republican Army was an Irish republican revolutionary paramilitary organisation. The ancestor of many groups also known as the Irish Republican Army, and distinguished from them as the "Old IRA", it was descended from the Irish Volunteers, an organisation established on 25 November 1913 that staged the Easter Rising in April 1916. In 1919, the Irish Republic that had been proclaimed during the Easter Rising was formally established by an elected assembly, and the Irish Volunteers were recognised by Dáil Éireann as its legitimate army. Thereafter, the IRA waged a guerrilla campaign against the British occupation of Ireland in the 1919–1921 Irish War of Independence.