The Irish Republic (Irish: Poblacht na hÉireann or Saorstát Éireann) was a revolutionary state that declared its independence from Great Britain in January 1919. It established a legislature (Dáil Éireann), a government (Aireacht), a court system and a police force. At the same time, the Irish Volunteers, who came under the control of the Dáil and became known as the Irish Republican Army, fought against British armed forces in the Irish War of Independence.

The War of Independence ended with the Anglo-Irish Treaty, signed on 6 December 1921 and narrowly approved by Dáil Éireann on 7 January 1922. A Provisional Government was set up under the terms of the treaty, but the Irish Republic nominally remained in existence until 6 December 1922, when Ireland became a self-governing British Dominion called the Irish Free State. The six counties of Northern Ireland exercised its right under the Treaty to opt out of the new dominion and rejoin the United Kingdom on 8 December 1922, leading to the Partition of Ireland, so that the Irish Free State consisted of only 26 of the island's 32 counties.