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.
History of the formation of the United Kingdom
The Act of Union 1800 united the Kingdom of Great Britain with the Kingdom of Ireland, which had been gradually brought under English control between 1541 and 1691, to form the United Kingdom of Great Britain and Ireland in 1801. Independence for the Irish Free State in 1922 followed the partition of the island of Ireland two years previously, with six of the nine counties of the province of Ulster remaining within the UK, which then changed to the current name in 1927 of the United Kingdom of Great Britain and Northern Ireland.