On 3rd May 1921 following the Government of Ireland Act of 1920, the six North-eastern counties of Derry, Antrim, Down, Armagh, Tyrone and Fermanagh were established as a separate entity, becoming Northern Ireland.
It was intended that Northern Ireland and the remaining 26 counties of Southern Ireland would be ruled as two separate territories but remaining part of the United Kingdom.
In 1922 following the War of Independence and the Anglo Irish Treaty, Southern Ireland became the Irish Free State, with Northern Ireland opting out of the new dominion and choosing to remain part of the United Kingdom.