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.
Partition of Ireland
Soon after the end of the war Sinn Féin won the overwhelming majority of the Irish parliamentary seats in the UK general election of 1918, and in January 1919 the Sinn Féin members declared unilaterally an independent (all-island) Irish Republic. Unionists, however, won a majority of seats in four of the nine counties of Ulster and affirmed their continuing loyalty to the United Kingdom. Following the Paris Peace Conference, in September 1919 Lloyd George tasked the "Long Committee" to implement Britain's commitment to introduce Home Rule, which was based on the policy of Walter Long, and some findings of the Irish Convention. The result was to be two home-rule Irish jurisdictions, and in November 1920 the Government of Ireland Act 1920 was enacted. As a result of this, in April 1921 the island was partitioned into Southern and Northern Ireland.