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 Ireland
The period 1916–1921 was marked by political violence and upheaval, ending in the partition of Ireland and independence for 26 of its 32 counties. A failed militant attempt was made to gain separate independence for Ireland with the 1916 Easter Rising, an insurrection in Dublin. Though support for the insurgents was small, the violence used in its suppression led to a swing in support of the rebels. In addition, the unprecedented threat of Irishmen being conscripted to the British Army in 1918 (for service on the Western Front as a result of the German Spring Offensive) accelerated this change. In the December 1918 elections Sinn Féin, the party of the rebels, won three-quarters of all seats in Ireland, twenty-seven MPs of which assembled in Dublin on 21 January 1919 to form a 32-county Irish Republic Parliament, the first Dáil Éireann unilaterally declaring sovereignty over the entire island.
Unwilling to negotiate any understanding with Britain short of complete independence, the Irish Republican Army, the army of the newly declared Irish Republic, waged a guerilla war (the Irish War of Independence) from 1919 to 1921. In the course of the fighting and amid much acrimony, the Fourth Government of Ireland Act 1920 implemented Home Rule while separating the island into what the British government's Act termed "Northern Ireland" and "Southern Ireland". In July 1921 the Irish and British governments agreed to a truce that halted the war. In December 1921 representatives of both governments signed an Anglo-Irish Treaty. The Irish delegation was led by Arthur Griffith and Michael Collins. This abolished the Irish Republic and created the Irish Free State, a self-governing Dominion of the Commonwealth of Nations in the manner of Canada and Australia. Under the Treaty, Northern Ireland could opt out of the Free State and stay within the United Kingdom: it promptly did so. In 1922 both parliaments ratified the Treaty, formalising independence for the 26-county Irish Free State (which renamed itself "Ireland" in 1937, and declared itself a republic in 1949); while the 6-county Northern Ireland, gaining Home Rule for itself, remained part of the United Kingdom. For most of the next 75 years, each territory was strongly aligned to either Catholic or Protestant ideologies, although this was more marked in the six counties of Northern Ireland.