Michael Collins is shown wearing a wrist watch. These were developed specifically for World War I airplane fighter pilots. It is far more likely that Collins would have carried a pocket watch during the period the film covers.
The car that de Valera gets into after arriving by boat has the registration 15 D 1, which indicates that although it is a 1915 car, it was imported and registered in Dublin as a historic vehicle sometime after 1987. An original Dublin registration of the time would be in the format IK 1234.
One of the sailing vessels pictured in the background as Collins, Boland, and Kitty Kiernan walk along the pier in the Dun Laoghaire Harbour is a Dublin Bay Sailing Club Mermaid. This class of 3-man racing dinghy was designed in 1932, 9 years after Collins' death.
When Michael Collins returns from London after the negotiation, he rides a car with Harry Boland and the head of a woman can be seen through the back window. The woman walks at the same speed as the car and remains seen in the same corner of the window, even though the car is increasingly speeding throughout the shot.
During the Easter Rising scenes, the Volunteers and Citizen Army are shown marching out of the General Post Office to surrender. However, the day before the surrender, they had retreated from the burning GPO to another building down the road, and surrendered from there. The white flag of surrender was actually displayed at 16 Moore Street, in another part of Dublin, where the leadership was residing.
When the cabinet are trying to convince Collins to engage in civil war Arthur Griffith says 'They have occupied the GPO, O'Connell Street, Limerick....'. O'Connell Street was only so named in 1924. In 1922 when this takes place it was still called Sackville Street.
Eamonn De Valera is shown surrendering with the General Post
Office garrison after the Easter Rising. However, he was actually Commandant of the garrison at Bolland's Mills, which surrendered after the GPO upon receiving orders to stand down. He was never at the GPO during the Rising.
There is a speech de Valera makes to a crowd after the treaty with the UK government is signed. He states in his fiery speech that "only pure blood Irish" should be able to participate in the new Republic, but he would have not said this since his father and surname were Spanish, a fact which his political opponents would use against him.
During the Easter Rising scenes, Collins is shown wearing the uniform of a full Lieutenant in the Irish Volunteers. While there is a photo of him in his Volunteers uniform as a Lieutenant, by the Easter Rising he had been promoted to Captain, where he served as aide de camp to Joseph Plunkett, who is conspicuously absent.
(at around 7 mins) We see Collins and Boland on a train, and it says May 1918. They are going to a rally for a by-election, we see the famous "Put him in to get him out" election posters from the 10 May 1917 by-election in South Longford where Joseph McGuinness was the SF candidate. The road sign says Longford, there were no by-elections in May 1918, and McGuinness was the only prisoner candidate in the 1917-18 by-elections. So it is clear that it should be May 1917 and not May 1918.
The film's introduction states that the United Kingdom was the foremost world power at the beginning of the 20th century. However this is not true as the UK was being surpassed economically and industrially by both Germany and the United States. In addition the Imperial German Navy had almost eclipsed the Royal Navy in size, threatening the UK's ability to hold onto its Empire.
The goof item below may give away important plot points.
Incorrectly regarded as goofs
In the film, it shows Ned Broy (Stephen Rea), a double agent in Dublin Castle being tortured and murdered by the British. While Broy survived the Irish Civil War in real life, the Broy in the film is a composite of both Broy and Dick McKee. McKee was killed in Dublin Castle reputedly whilst attempting to escape custody after being captured before Bloody Sunday.