The Partition of Ireland took place on 3 May 1921 under the Government of Ireland Act 1920. The entire island of Ireland provisionally became the Irish Free State on 6 December 1922. However, the Parliament of Northern Ireland exercised its right to opt out of the new Dominion the following day.

Partition created two territories on the island of Ireland: Northern Ireland and Southern Ireland. Today the former is still known as Northern Ireland while the latter is known simply as Ireland (or, if differentation between the state and the whole island is required, the state can be referred to as the Republic of Ireland).