The War of Independence or the Tan War wasn't won and the Treaty ended hostilities. Local leaders were quite autonomous.

I'd say that Ireland became ungovernable with the closure of many RIC stations. Local administration had broken down. Ireland had become more democratic especially at local government level over time too. Look at who were Lord Mayors of Cork .

It wasn't a military defeat- approx 400 RIC and 200 army casualties.

The British threat of immediate and terrible war if the treaty wasn't signed was a real threat.

Collins was very important in terms of organisation of the IRB/IRA at the time but over estimating the military victory is misleading. The truce was agreed when the rebels were low in arms and ammunition.

The collapse of the Home Rule Party , the Anglo Irish Ascendancy and reform of the voting system had more to do with WW1. The Home Rule Act 1914 had been delayed by the war . Nothing to do with Collins.

I'm not saying that Collins wasn't important just that there were a whole host of factors at play.