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.
Only 3 of the main actors are Irish or of Irish descent, those being, Fionnula Flanagan, Brían F. O'Byrne, and Fiona C. Erickson (whose maternal grandfather was born and raised in Westport, Co. Mayo).
Kevin Chapman who plays Freddie Cork was discovered by the late Director Ted Demme while working in the City Of Boston Film Office.
Al Sapienza and Matt Servitto and John Fiorre are the only three actors on the show who were also on "The Sopranos".
Features Janel Moloney's first nude scene.