Despite containing several brutal scenes of violence, the film was given a very lenient 'PG' rating in Ireland mainly because of its historical context. The censor issued a press statement defending his decision claiming the film was a landmark in Irish cinema and that he believed "because of the subject matter, parents should have the option of making their own decision as to whether their children should see the film or not". The Film subsequently became the second most successful movie ever released in Ireland.
Tom Cruise was offered the 'Jonathan Rhys-Meyers' cameo of the Assassin
This was originally going to be a Kevin Costner feature.
The final piece of score by Elliot Goldenthal in the film, "Funeral/Coda," is actually Goldenthal's rejected finale for Heat (1995), which director Michael Mann replaced with an existing Moby piece.
The character Ned Broy, played by Stephen Rea in the film, is in fact a composite of the historical figures Ned Broy, who was a double agent in the police, and Dick McKee, who was Commandant of the Dublin Brigade of the IRA and hence a central figure in planning intelligence operations with Collins. Broy survived the war, but McKee was captured the night before the attack on British agents, tortured and shot.
Director Jordan shows an armored vehicle with a machine gun turret shooting at players and spectators during the Croke Park Massacre in Dublin during a Gaelic Football match. In reality the Black and Tans only used trucks and rifles.
Liam Neeson was 43 when this movie was made. The real-life Michael Collins was 31 when he died.
Bray Wanderers' Carlisle Grounds substituted for 1920's Croke Park, at a cost of approx IRL£1 million to renovate the ground accordingly.
Brendan Gleeson, who plays Liam Tobin in the film, played Collins in The Treaty (1991) (TV). Liam Neeson consulted with Gleeson during pre-production and on set on portraying the character.