Great piece on the Anglo-Irish Treaty of 1921 by John Dorney over at my newer (and better treated website) The Irish Story:
By the time of the Treaty negotiations, the partition of Ireland was therefore an established fact and no longer up for negotiation. Thus the Unionists, under James Craig, did not even take part in the Treaty talks. The Sinn Fein delegation insisted that they could not accept a settlement that made partition permanent, but the only element of the northern situation to be seriously discussed was the future of counties Fermanagh and Tyrone, both of which had Catholic majorities. The Irish wanted a county by county referendum on inclusion into the northern or southern states.
What they got in the end was that Northern Ireland as a whole was given the option of uniting with the southern state after a year. There would also be a Border Commission set up to arbitrate on how the border could be changed to reflect the wishes of the local population. It was the hope of Irish delegation that Northern Ireland’s viability would eventually be undermined by the defection of much of its Catholic-populated western and southern territory to the southern state. Nevertheless, the Treaty confirmed the partition of Ireland in the short term.