Factual error: On the very beginning of the movie, as well as few times throughout, we can see shots of GPO (General Post Office) taken from the street right in front of it. There is no, and never was, a street there. It's about hundred meters to the right, and you can't see (nor bombard) the GPO from it.
Continuity mistake: When Michael Collins and his men raid the police station they throw torches on the roof. A second later the whole building is on fire, and the policemen run out coughing and gasping for air. However, when the IRA people go in to get the police guns the air is quite breathable. (00:15:25)