ok more details.
the Home Rule bill of 1914 did pass, however the out brake of WWI put implementing of the law off, support for an out and out Republic just wasn't there in 1914, the famous Easter Rising also had very very little support, what really set off the Sinn Féin election victory was anger over the conscription of the Irish in 1918 (there was nothing gained by conscription as the Irish volunteered at such high rates any ways) the war was badly managed by the Brits, by the time peace talks started the IRA was on the ropes (the IRA's leader, Michael Collins, said they only had about enough bullets for a week of fighting at the end) many believed that the Anglo-Irish Treaty was a ploy by the British who believed the Irish would say no to it and thus the Brits would have a reason to impose Martial law on Irish, the Treaty nearly did fail (passed the Irish by 7 votes or so) even after it passed about half the IRA lead by Éamon de Valera said they'd keep fighting, they took parts of Dublin (the Four Courts) and else where in Ireland, there was a months long stand off as the Free State Government lead by Collins was unwilling to start a civil war and the IRA under de Valera was unwilling to back down, it all came to a head because of Collins' "Northern offensive" where the IRA Belfast Brigade lunched attacks in the North during may 1922 under Collins' orders, Collins also had the IRA kill Field Marshal Sir Henry Hughes Wilson in London itself, Winston Churchill than Secretary of State for the Colonies, thought the London hit was de Valera and the anti-Treaty boys, he told Collins to take them out, or the Brits would.
any ways there are lots of chances for the Irish to fail to win themselves a country and in those cases I could see a large number of abstentionist Sinn Féin members, though 90 years is a long time to hang on to something, and give up your right to a say in government.