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.
United Kingdom of Great Britain and Ireland
Growing desire for Irish self-governance led to the Irish War of Independence, which resulted in most of Ireland seceding from the Union and forming the Irish Free State in 1922. Northern Ireland remained part of the Union, and the state was renamed to the current "United Kingdom of Great Britain and Northern Ireland" in 1927. The modern-day United Kingdom is the same country as the one from this period—a direct continuation of what remained after the secession—not an entirely new successor state.