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.
Partition of Ireland
The Government of Ireland Act 1920, which came into effect on 3 May 1921, provided for separate self-governing parliaments for Northern Ireland (the six northeastern counties) and Southern Ireland (the rest of the island), thus partitioning Ireland. While the parliament and governmental institutions for Northern Ireland were soon established, the election in the 26 counties returned an overwhelming majority of members giving their allegiance to Dáil Éireann and supporting the republican effort in the Irish War of Independence, thus rendering "Southern Ireland" dead in the water.