The Federal government of the U.K. of Great Britain and Ireland segmented Ireland into 2 independent polities: Northern Ireland and Southern Ireland. The Federal Government of Ireland Act 1920 was handed down May third, 1921. Both parts were to stay inside the UK, and the Act included arrangements for supreme reunification.
Northern Ireland was established with a devolved federal government and stayed a part of the U.K. Most occupants of Southern Ireland didn't recognize the self-proclaimed Irish Republic, choosing to recognize the larger Northern Ireland. The area of Southern Ireland withdrew from the U.K. and ended up being the Irish Free State, which is today the Republic of Ireland, following the Anglo-Irish Treaty.
In this book, we will look at further details of the partition of Ireland.