Factual error: During one of the cabinet meetings, one of the members says Collins' brief is Minister for Intelligence, but he was actually Minister of Finance. He was, however, the IRA's Director Of Intelligence.
Factual error: The Lee Enfield rifle is chambered by singular repetitions with a bolt action. When Collins' wayward friend is shot in the water, the rifle shots are rapid semi-automatic fire.
Factual error: During the Croke Park massacre, the British never rolled an armored car onto the Gaelic football field, as depicted in the film. It remained outside the gates. (01:05:20)
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.
Factual error: The IRA did not start to use car bombs until the 1970's.
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. (00:46:05)
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.
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.