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 the formation of the United Kingdom
The Act of Union 1800 united the Kingdom of Great Britain with the Kingdom of Ireland, which had been gradually brought under English control between 1541 and 1691, to form the United Kingdom of Great Britain and Ireland in 1801. Independence for the Irish Free State in 1922 followed the partition of the island of Ireland two years previously, with six of the nine counties of the province of Ulster remaining within the UK, which then changed to the current name in 1927 of the United Kingdom of Great Britain and Northern Ireland.