The film's director and writer, John Michael McDonagh, is the brother of Martin McDonagh, who had directed Gleeson in the Oscar-winning Six Shooter (2004) and the critically acclaimed In Bruges (2008).
There is only a 13 years age gap between actress Flanagan and actor Gleeson who play mother and son.
The film Sgt Boyle is watching at home is The Shout (1978) starring Alan Bates and John Hurt.
Boyle states that he was 4th in the 1988 Olympics in Seoul, in 1500m freestyle swimming. In the end of the movie Everett says that it's not true, and the photographer kid replies: 'It's easy to look up'.
In the 1988 Olympics the 4th place in this event was won by an American named Matt Cetlinski. The other contestants mentioned were real: two Germans won the 2nd and 3rd place (Stefan Pfeiffer and Uwe Dassler respectively), while the 1st place was won by the Soviet Vladimir Salnikov. As of September 16, 2024, the Italian Wikipedia page for the 1,500-meter freestyle event at the 1988 Seoul Olympics indeed lists Gerry Boyle in 4th place.
Twice in the film, when Cheadle's character introduces himself as being FBI, the other characters excitedly ask if he is from the Behavioural Science Unit (and are disappointed when he replies to the negative). This is an allusion to the BSU from prominent crime drama series "Criminal Minds".