The Troubles
The Government of Ireland Act 1920 partitioned the island of Ireland into two separate jurisdictions, Southern Ireland and Northern Ireland, both devolved regions of the United Kingdom. This partition of Ireland was confirmed when the Parliament of Northern Ireland exercised its right in December 1922 under the Anglo-Irish Treaty of 1921 to ""opt out"" of the newly established Irish Free State. A part of the treaty signed in 1922 mandated that a boundary commission would sit to decide where the frontier of the northern state would be in relation to its southern neighbour. After the Irish Civil War of 1922–1923, this part of the treaty was given less priority by the new Dublin government led by W. T. Cosgrave, and was quietly dropped. As counties Fermanagh and Tyrone and border areas of Londonderry, Armagh, and Down were mainly nationalist, the Irish Boundary Commission could reduce Northern Ireland to four counties or less.
Irish Republic
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 26 of the island's 32 counties became a self-governing British Dominion called the Irish Free State. The island had been partitioned by the Government of Ireland Act 1920, and the six counties of Northern Ireland exercised their right under the Treaty to opt out of the Free State, and remain in the United Kingdom.