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 border question
In 1922, the Irish Free State repatriated its national legislature from London, setting the stage for full national independence, while Northern Ireland remained under the governance of the United Kingdom. Consequently the dividing line between these two parts of the island became an international border. Trade in goods and services across this frontier became subject to differing tax and tariff arrangements and an infrastructure of Customs posts was put in place at designated crossing areas. All traffic was subject to inspection by the jurisdiction it was entering. This could entail full vehicle searches with consequent delay and inconvenience. However, passport checks were not applied because the Republic of Ireland and Northern Ireland were part of the Common Travel Area.