Revealing mistake: In two scenes - first when Harry Boland is getting on a train, and next during the game which is machine gunned by the authorities, overhead DART (Dublin Area Rapid Transit) electric train lines are visible over the railway. The film is set between 1916 and 1922, and the DART started operations in 1984.
Revealing mistake: There's one head of police who, during his short stint in Dublin, pledges to introduce 'Belfast efficiency'. Shortly after that he is killed by a car bomb. However, when the bomb goes off there's a split-second shot where the car is empty although three men had just gotten in, and the driver who was starting the car is a rather crude dummy.
Continuity mistake: There is a scene in which two Dublin United Tramways Corporation trams pass in opposite directions. Neither has its trolley pole in contact with the overhead electrical wiring, so the electricity must be jumping the gap to the motor in some way unknown to physics! The trams were, of course, fakes with a petrol engine.
Factual error: Michael Collins recites a quote and his friend asks who said that. Collins says, "Him. Peter Pan," and closes a book. The "him" was obviously referring to the character in the present book: a very thick hardback. However, both "Peter Pan and Wendy" and "Peter Pan" are very thin. This almost certainly would not have been a collection of works either considering how recently Peter Pan would have come out relative to the movie's period.
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.