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 quad poster used to advertise the film in Ireland carried a warning message that read, "WARNING TO PARENTS AND GUARDIANS: This film includes scenes depicting explicit cruelty and violence along with crude language. It is advised that children under 12 years be accompanied by a parent or adult guardian". The film subsequently became the second most successful movie ever released in Ireland.
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 and shot, reputedly whilst attempting to escape. Broy, only a Sergeant in the Dublin Metropolitan Police would be rewarded by being made head of the Irish Free State's new police national force, the Garda. Ironically he would go to great lengths to encourage ex-Royal Irish Constabulary officers to join the new force, giving them preference in recruitment and absorb the Dublin Metropolitan Police as a whole into the new organization.
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 Royal Irish Constabulary only used trucks and rifles.
Recently released documents show that Michael Collins used former Black and Tans against the IRA during the Battle of Dublin (28 June-5 July 1922). It is unclear whether Collins requested these reinforcements, or whether the UK government provided them because they did not believe the National Army was capable of driving the anti-Treaty forces out of Dublin.
The trivia items below may give away important plot points.
The later-on released DVD was flippable and had two sides, much like an old LP. In one of the first scenes, Michael Collins fought along with the rebellion against the military. After you flip over the DVD, the first scene shown is Collins now on the military side fighting against the new-formed rebellion. The side-change of the DVD dramatically marks the change of Collins' position.
This film did not show Eamon De Valera ordering the assassination of Collins but it did insinuate that he ordered it. This has been a highly contentious issue in Ireland ever since; although De Valera was widely believed to have ordered the murder he always denied it.
It could be considered a plausible theory that the unnamed Jonathan Rhys Meyers character, represents the post-Free State founding, and later versions of, the "New" IRA method of using the gun in politics to get their way, replacing Michael Collins' more cerebral negotiations, with a gun as a last resort "original" IRA methodology.