Britain had been using military force in Ireland for centuries.
Very true but given the increase in the reporting and quite frankly the level of violence that the War of Independence created it was different from previous times.
I don't see how you get to 1921 and expect Ireland to remain within the UK, both the active campaign and the resulting votes (and the 1918 votes) show that it had gone to far. The UK would have had to garrison the majority of the island with much more force than historically deployed in order to maintain control. They could have done so, but it wouldn't have been anything close to choosing to remain within the Union.