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.
Counties of Ireland
Under the Government of Ireland Act 1920, the island was partitioned between Southern Ireland and Northern Ireland. For the purposes of the Act, ... Northern Ireland shall consist of the parliamentary counties of Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone, and the parliamentary boroughs of Belfast and Londonderry, and Southern Ireland shall consist of so much of Ireland as is not comprised within the said parliamentary counties and boroughs.