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.
Counties of Northern Ireland
The English administration in Ireland in the years following the Anglo-Norman invasion of Ireland created counties as the major subdivisions of an Irish province. This process lasted a period from the 13th to 17th centuries, however the number and shape of the counties that would form the future Northern Ireland would not be defined until the Flight of the Earls allowed the shiring of Ulster from 1604.