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.