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 The latter were withdrawn soon afterwards upon the establishment of the Provisional Government, but the former continued to issue revenue stamps until the late 20th century.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.
Ireland
In December 1921, the Anglo-Irish Treaty was concluded between the British government and representatives of the Second Dáil. It gave Ireland complete independence in its home affairs and practical independence for foreign policy, but an opt-out clause allowed Northern Ireland to remain within the United Kingdom, which it immediately exercised as expected. Additionally, Members of the Free State Parliament were required to swear an oath of allegiance to the Constitution of the Irish Free State and make a statement of faithfulness to the King. Disagreements over these provisions led to a split in the nationalist movement and a subsequent Irish Civil War between the new government of the Irish Free State and those opposed to the treaty, led by Éamon de Valera. The civil war officially ended in May 1923 when de Valera issued a cease-fire order.