Britsh intervention led to the Irish civil war, also both sides of the Sinn Fein split were due to divy out government ministerial posts to each other prior to the civil war and it didn't help that Collins, the big fella reneged on this one! The Brits would never tolerate an anti-treaty victory, which could never have happened anyway because the pro-treatyites knew where all the safe houses were! BTW had the Jaffa Statelette ever become a more serious ethnic cleansing issue than it was, the Brits would also substantially intervene as it could not tolerate a Bosnia on its doorstep!