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)
Factual error: The IRA did not start to use car bombs until the 1970's.
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.