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 Boundary Commission
The Government of Ireland Act 1920 was enacted during the height of the War of Independence and partitioned the island into two separate Home Rule territories of the United Kingdom of Great Britain and Ireland, to be called Northern Ireland and Southern Ireland. In its determination of this border, the Parliament of the United Kingdom heard the arguments of the Irish Unionist Party, but not those of most of the elected representatives of the Irish nationalist population. Sinn Féin refused to recognise any legitimate role of that Parliament in Irish affairs and declined to attend it, leaving only the Irish Parliamentary Party, whose representation at Westminster had been reduced to miniscule size at the 1918 General Election, present at the debates. James Craig's brother told the British House of Commons unambiguously that the six northeastern counties were the largest possible area that unionists could "hold".