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
In May 1949 the Taoiseach John A. Costello introduced a motion in the Dáil strongly against the terms of the UK's "Ireland Act 1949" that confirmed partition for as long as a majority of the electorate in Northern Ireland wanted it, styled in Dublin as the "Unionist Veto". This was a change from his position supporting the Boundary Commission back in 1925, when he was a legal adviser to the Irish government. A possible cause was that his coalition government was supported by the strongly republican Clann na Poblachta. From this point on all the political parties in the Republic were formally in favour of ending partition regardless of the opinion of the electorate in Northern Ireland.