Irish Boundary Commission

The Irish Boundary Commission (Irish: Coimisiún na Teorann) met in 1924–25 to decide on the precise delineation of the border between the Irish Free State and Northern Ireland. The 1921 Anglo-Irish Treaty, which ended the Irish War of Independence, provided for such a commission if Northern Ireland chose to secede from the Irish Free State (Clause or Article 12), an event that occurred as expected two days after the Free State's inception on 6 December 1922, resulting in the partition of Ireland.

Source: Wikipedia — Irish Boundary Commission (CC BY-SA 4.0)