Continuity mistake: Early in the film, Michael talks to Harry Boland on a boat. During this scene, a cigarette appears in Boland's mouth that was not in the previous shot. (00:40:40)
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 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.
Revealing mistake: When Liam Neeson is on the boat on his return from England there is a shot of the water and part of the boat where you can see a modern CCTV camera on the side of one of the masts - rather advanced for the time the film was set. (01:21:20)
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: After the attack on the ball game Michael Collins visits a morgue. The corpse that is lying next to him is breathing quite visibly. (01:07:20)
Other mistake: When Kitty reads Michael's letter from England to Harry, there's some strange smoke arising from behind their chairs. (01:19:40)
Continuity mistake: When Michael Collins searches through the files in the castle he takes the only photograph from his file and shows it later to Harry Boland. But when the file is given to Soames, the photograph is back in. (00:30:30 - 00:52:15)
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: When Soames gets shot through a folding screen huge fireballs erupt from his body when the bullets hit him. (01:03:30)
Revealing mistake: In the first scene, when the IRA fighters in the burning building surrender, the absolutely smoke-free fires in the windows look like cosy, gas-fuelled fireplaces. (00:03:50)
Continuity mistake: When Michael Collins joins the first cabinet meeting after DeValera's return he puts a briefcase on the table in front of him and takes out some papers. When the camera angle changes the briefcase is gone. (01:09:45)
Continuity mistake: There's one scene where Michael Collins talks to Ned Broy standing on a small pedestrian bridge while a train passes overhead. From the railing construction you can tell that the close-up was taken at a different place than the wide shot. (00:51:05)
Factual error: The IRA did not start to use car bombs until the 1970's.
Continuity mistake: When DeValera returns from America Joe O'Reilly hands him a paper with the news. It is folded in a way that shows cover outside, but when DeValera opens it the crease is the other way. (01:08:45)
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)
Continuity mistake: When Michael Collins announces to his men that all collaborators will be shot his hair changes from messy to perfectly combed between shots. (00:30:50)
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.
Other mistake: After the attack on the police station we see a train passing by in front of the fire. The shiny train reflects the flames in a way that could only be caused by another fire for which there is no evidence. (00:16:20)
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.