Revenue stamps of the United Kingdom
When Ireland was partitioned into Northern Ireland and Southern Ireland in 1921, separate revenue stamps were issued for both entities. Southern Ireland stamps saw little use since that political entity was quickly superseded by the Provisional Government of Ireland and later the Irish Free State (both of which applied overprints on various British and Irish revenue stamps). However, Northern Ireland continued to use separate revenue stamps until the 1980s. Northern Irish revenues include Civil Service, Dog Licence, Excise and National Insurance stamps, among many others.
Royal and Parliamentary Titles Act 1927
As a result of the Anglo-Irish Treaty, in 1922 most of Ireland was detached from the United Kingdom of Great Britain and Ireland to become the Irish Free State. However, six north-eastern counties remained united with Great Britain as Northern Ireland.