Two 'Home Rule' Irelands
The Act divided Ireland into two territories, Southern Ireland and Northern Ireland, each intended to be self-governing, except in areas specifically reserved to the Parliament of the United Kingdom: chief amongst these were matters relating to the Crown, to defence, foreign affairs, international trade, and currency.
"Southern Ireland" was to be all of Ireland except for "the parliamentary counties of Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone, and the parliamentary boroughs of Belfast and Londonderry" which were to constitute "Northern Ireland". Northern Ireland as defined by the Act, amounting to six of the nine counties of Ulster, was seen as the maximum area within which Unionists could be expected to have a safe majority. This was in spite of the fact that counties Fermanagh and Tyrone had Catholic Nationalist majorities.
Read more about this topic: Government Of Ireland Act 1920