The Troubles
The Government of Ireland Act 1920 partitioned the island of Ireland into two separate jurisdictions, Southern Ireland and Northern Ireland, both devolved regions of the United Kingdom. This partition of Ireland was confirmed when the Parliament of Northern Ireland exercised its right in December 1922 under the Anglo-Irish Treaty of 1921 to ""opt out"" of the newly established Irish Free State. A part of the treaty signed in 1922 mandated that a boundary commission would sit to decide where the frontier of the northern state would be in relation to its southern neighbour. After the Irish Civil War of 1922–1923, this part of the treaty was given less priority by the new Dublin government led by W. T. Cosgrave, and was quietly dropped. As counties Fermanagh and Tyrone and border areas of Londonderry, Armagh, and Down were mainly nationalist, the Irish Boundary Commission could reduce Northern Ireland to four counties or less.
Irish migration to Great Britain
Irish migration to Great Britain has occurred from the earliest recorded history to the present. There has been a continuous movement of people between the islands of Ireland and Great Britain due to their proximity. This tide has ebbed and flowed in response to politics, economics and social conditions of both places. Ireland was a feudal Lordship of the Kings of England between 1171 and 1541; a Kingdom in personal union with the Kingdom of England and Kingdom of Great Britain between 1542 and 1801; and politically united with Great Britain as the United Kingdom of Great Britain and Ireland between 1801 and 1922. Today, Ireland is divided between the independent Republic of Ireland and Northern Ireland.