Veronica Guerin attends a protest march organized by he Concerned Parents Against Drugs to pressure a drug dealer to leave the area, and behaves as if this is a new tactic. In fact, the CPAD marches had begun in 1983 (12-13 years before the film's events) and were not a new phenomenon at the time.
When Traynor meets with Veronica on a pier in Dublin, he parks his car right behind Veronica's car, between two small pillars. When their meeting is over, Veronica drives away, which should be impossible because the exit off the pier is completely blocked by Traynor's car and the pillars.
At one point, Veronica is working at home, wearing headphones while listening to music. Her husband asks her to come to bed. Veronica takes of her headphones to chat with him. In the next shot, when they are dancing, she is wearing the headphones again.
(at around 1h 25 mins) When we see Veronica through the sunroof of her car after she is shot, a hand is visible in the lower right corner of the sunroof, moving in and out of the shot. It appears to be a crew member's hand.
In one of the night shots of the exterior of Veronica's home, an owl can be heard 'hooting' in the distance. This is the call of the Tawny owl, which although common in Britain, is not found in Ireland.
During the epilogue, it is claimed that in an emergency Parliament session, the Government altered the Constitution. This is inaccurate for two reasons. Firstly, no such amendment of the Constitution occurred. Secondly, when an amendment is made, the Government alone does not have the authority to enact it: it may only propose such amendments to the people, in the form of a referendum.