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.
History of Northern Ireland
Northern Ireland is one of the four countries of the United Kingdom (although it is also described by official sources as a province or a region), situated in the northeast of the island of Ireland. It was created as a separate legal entity on 3 May 1921, under the Government of Ireland Act 1920. The new autonomous Northern Ireland was formed from six of the nine counties of Ulster: four counties with unionist majorities and two counties, Fermanagh and Tyrone, which had slight Irish nationalist majorities (of 53.6 and 54.6 per cent respectively in the 1918 election). The remaining three Ulster counties with larger nationalist majorities were not included. In large part unionists, at least in the northeast, supported its creation while nationalists were opposed.