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 republicanism
In 1948, Fianna Fáil went out of office for the first time in sixteen years. John A. Costello, leader of the coalition government, announced his intention to declare Ireland a republic. The Republic of Ireland Act 1948, which "described" the state as the Republic of Ireland (without changing its name), led the British government to pass the Ireland Act 1949, which declared that Northern Ireland would continue as part of the United Kingdom unless the Parliament of Northern Ireland consented to leave; and Ireland ceased to be a member of the Commonwealth. As a result of this—and also because continuing struggle against the Dublin government was futile—the republican movement took the decision to focus on Northern Ireland from then on. The decision was announced by the IRA in its Easter statement of 1949.