The series was inspired by real-life Irish American brothers from the Irish neighborhood of South Boston: James "Whitey" Bulger, Jr., a gangster who has been on the F.B.I.'s Most Wanted List for 10 years as a fugitive from justice, and William Bulger (known as "Billy"), a powerful state politician who was President of the Massachusetts Senate and later president of the University of Massachusetts higher educational system.
Kevin Chapman who plays Freddie Cork was discovered by the late Director Ted Demme while working in the City Of Boston Film Office.
Only 3 of the main actors are Irish or of Irish descent, those being, Fionnula Flanagan, 'Brían O'Byrne', and Fiona Erickson (whose maternal grandfather was born and raised in Westport, Co. Mayo).
Series star Len Cariou and guest star Kevin Conway appeared together in the Cuban Missile Crisis film Thirteen Days.
SPOILER: When talking inside the diner while making their escape from Michael (Jason Isaacs), Kath ('Tina Banko') mentions to Colin ('Brían F. OByrne) she wants to move to Missoula, Montana. The city of Missoula is the hometown of writer/director/painter David Lynch, famous for his often surreal and enigmatic imagery along with very graphic violence, in reference to Michael's erratic, violent and deepening paranoid behaviour.