Revenue stamps of Northern Ireland
Upon the partition of Ireland in 1921, separate revenue stamps began to be issued for both Northern Ireland and Southern Ireland. The latter were withdrawn soon afterwards upon the establishment of the Provisional Government, but the former continued to issue revenue stamps until the late 20th century. The need for separate issues for Northern Ireland came about since taxes raised there were meant to remain in the country. Most Northern Irish revenue stamps were British key types with appropriate inscriptions, but there were a number of different issues as well.
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.