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.
Northern Ireland
Events overtook the government. The pro-independence Sinn Féin won 73 of the 105 parliamentary seats in Ireland at the general election of 1918, and unilaterally established the First Dáil, an extrajudicial parliament in Ireland. Ireland was partitioned between Northern Ireland and Southern Ireland in 1921, under the terms of Lloyd George's Government of Ireland Act 1920, during the Anglo-Irish War between Irish republican and British forces. A truce was established on 11 July; the war ended on 6 December 1921 with the signing of the Anglo-Irish Treaty, which created the Irish Free State. Under the terms of the treaty, Northern Ireland would become part of the Free State unless the government opted out by presenting an address to the king, although in practice partition remained in place.