Based on a novel 'Bad day in Blackrock' which was itself at least partially inspired by real live events, concerning what became known as the Anabel's night club murder in Dublin in 2000.
In the first iteration of the screenplay, there was no character of Jake. The production team loved Patrick Gibson's audition so much that the part was created for him.
Jack Reynor gained weight as filming progressed to show Richard's lack of control over himself following the incident that changed his life. Reynor's love of pies was instrumental in his physical change.
Lenny Abrahamson garnered most of his cast through improvisational workshops that he started a full year in advance of shooting.