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 issue in British politics
The Anglo-Irish War was fought between the forces of the provisional government of the Irish Republic and Britain, between January 1919 and July 1921. The British tried repressive force but the IRA under Michael Collins used about 3,000 fighters in asymmetric warfare. Prime Minister David Lloyd George presided over the Government of Ireland Act 1920 which partitioned Ireland into Southern Ireland and Northern Ireland in May 1921 during the Anglo-Irish War. Lloyd George used the threat of a massive war to force the Collins and his Irish negotiators to agree to the peace treaty in December 1922. It created a largely autonomous dominion, the Irish Free State, to replace the (failed) Southern Ireland. Meanwhile Northern Ireland, or Ulster, (with home rule) remained part of the United Kingdom. The Dail majority supported Collins and the treaty, but De Valera, President of the Dail, rejected it. De Valera led a civil war in Ireland that failed. The treaty with Britain went into effect and Northern Ireland remained with the UK.