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.
Ireland–United Kingdom relations
19th century violent and constitutional campaigns for autonomy or independence culminated in an election in 1918 returning almost 70% of seats to Sinn Féin, who declared Irish independence from Britain and set up a parliament in Dublin, and declared the independence of Ireland from the United Kingdom. A war of independence followed that ended with the Anglo-Irish Treaty of 1922, which partitioned Ireland between the Irish Free State, which gained dominion status within the British Empire, and a devolved administration in Northern Ireland, which remained part of the UK. In 1937, Ireland declared itself fully independent of the United Kingdom.