The house of writer and director Sir John Boorman was robbed by the real-life Martin Cahill. Among other things, he stole a gold record that Boorman had on the wall (an award for the "Dueling Banjos" theme from Délivrance (1972)), which inspired Boorman to include that scene in the movie.
When the film was shown in theaters, it was in black-and-white, but when it aired on American cable television networks, it was shown in color.
Although, as the film reflects, Martin Cahill would wear a balaclava or cover his face with his hands at public appearances when he would be photographed by the press, he would also pull back his outer clothes to reveal Batman T-shirts (or Mickey-Mouse-branded underwear).
The film doesn't make it clear, but Dr James Donovan, the forensics expert who Martin Cahill had car-bombed to stop him testifying, never recovered and was left disabled and in chronic pain. Also, though the film shows Cahill's rage at his unemployment benefits (which he continued drawing on top of his substantial criminal earnings) being stopped, it doesn't show the action he took (having the welfare inspector who dealt with his case shot in both legs).
Well known Irish director and writer Jim Sheridan has a small part in the movie as an anti-drug campaigner. He is the one challenging Adrian Dunbar's character.