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
It was not until the reign of Queen Elizabeth I that Ulster would be shired into more counties, based largely on the boundaries of existing lordships. In 1584, Lord Deputy of Ireland Sir John Perrott created the following counties in what is now Northern Ireland: Armagh; Coleraine; Fermanagh; and Tyrone. Carrickfergus would retain its status as a county town until 1777 when it was merged with County Antrim.