Republic of Ireland–United Kingdom border
Originally intended as an internal boundary within the United Kingdom of Great Britain and Ireland, the border was created in 1921 under the United Kingdom Parliament's Government of Ireland Act 1920. Prior to this, a separatist Irish parliament had been established in Dublin, which did not recognise the Government of Ireland Act, and was actively engaged in the Irish War of Independence. The Act was intended to deliver Home Rule in Ireland, with separate parliaments for Southern Ireland (which included three of the nine counties of Ulster) and Northern Ireland. Six of the thirty-two counties of Ireland were assigned to Northern Ireland, and the rest of Ireland comprising 26 counties to Southern Ireland.
Irish republicanism
In 1948, Fianna Fáil went out of office for the first time in sixteen years. John A. Costello, leader of the coalition government, announced his intention to declare Ireland a republic. The Republic of Ireland Act 1948, which "described" the state as the Republic of Ireland (without changing its name), led the British government to pass the Ireland Act 1949, which declared that Northern Ireland would continue as part of the United Kingdom unless the Parliament of Northern Ireland consented to leave; and Ireland ceased to be a member of the Commonwealth. As a result of this—and also because continuing struggle against the Dublin government was futile—the republican movement took the decision to focus on Northern Ireland from then on. The decision was announced by the IRA in its Easter statement of 1949.