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.
Economic history of Ireland
After the War of Independence, most of Ireland gained independence from the United Kingdom. Twenty-six counties of Ireland became the Irish Free State, later described as the Republic of Ireland, while the other six remained in the Union as Northern Ireland. There had already been a significant economic divide between these two parts of Ireland, but following partition both regions further diverged, with Belfast, as the North's economic centre, and Dublin becoming the capital of the Free State. Partition had a devastating effect on what became Ireland's border area. County Donegal for example was economically separated from its natural regional economic centre of Derry. The rail network struggled to operate across two economic areas, finally closing across a vast swath of Ireland's border area (the only cross-border route left being that between Belfast and Dublin). The last remaining cross-border line (Belfast-Dublin) could not have operated after 1953 without government support.