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.

The original Irish Republican Army (1919–1922), 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, Dáil Éireann, in 1919.

In the century that followed, the original IRA was reorganized, 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 1969-1998 Troubles in Northern Ireland. The contemporary IRA organisations each claim the sole right to the name, each insisting they are the original IRA's only legitimate descendant.