The Irish Independence war was by 1921 becoming increasingly unpopular in Britain, and many felt that the British were on the ropes, hence the treaty. Of course, the Irish Republicans had their own problems, mainly a shortage of arms and ammunition with which to continue the war. Whether or not the British knew this, however, is another question. Basically, what I'm wondering is: if Collins doesn't sign the treaty in 1922 can the Old IRA keep Britain in a costly enough stalemate to force their full demands for a united and independent Irish Republic?