After the character played by Liam Neeson retrieves his hunting gun on the back seat of the terrorists'car and sets to tracks Doireann the rear right door of the car behind him is seen opened with no reason.
When Liam Neesons character offers to let Curtis look at the shotgun before pistol whipping him, the shotgun changes from a sidelock engraved shotgun to a plain boxlock.
Briefly dropping a shotgun cartridge into water would not cause it to misfire.
During the opening couple minutes a Triumph 2500S is seen driving along a street and parking, the Triumph 2500S wasn't produced until 1975 so didn't exist in 1974 when this film is set,
At 6 minutes in, when Finbar and the Garda are discussing the broken sign, two wind turbines can be seen on a hill in the distance on the right-hand side of the screen. The 1st wind turbine in Northern Ireland wasn't until around 1995. The film is set in 1974.