Once upon a time, there was a fifth wheel to a group of friends from Queens. But that was before two of the guys in the group (brothers by the name of Chase) became actors in Hollywood.
That fifth wheel was named Dom and he ended up doing time just as the rest of the gang headed out to fame, if not fortune, in Hollywood.
After Dom got out of prison he tried to find a place for himself back in his former circle of friends. But in the five years that had passed, Vincent Chase and his entourage had grown beyond the experiences they once shared. Although all but Vince, who owed Dom a huge debt for taking the heat on a pot possession charge, had problems with Dom's interference in their group dynamic, it was Eric Murphy who was openly suspicious and hostile to his presence among them.
Eventually Dom got in trouble again with the law when he violated the terms of his parole. In the end, it was likely Dom understood the detrimental effect he had on his old friends. But that probably had little effect on the anger he must have felt towards Eric. Dom must have escaped from prison and headed back East. But instead of hanging out in his old haunts around Queens Boulevard, where the authorities would have been looking for him, Dom established a new base of operations in Brooklyn. There he fully embraced a life of crime as he had nothing left to lose - not only was he on the outs with his childhood friends, but his girlfriend had taken the baby and left him because he got in trouble with the law again.
Dom worked with someone else he knew from the old days named Jim. (It could even be that Jim was his brother; they do share something of a similarity in appearance.) They worked as the muscle for a dominatrix club which they were hoping to rip off (by stealing confidential information on the clients for potential blackmail purposes). And for that information, Dom was even willing to commit murder when a private eye named Jonathan Ames got to the disk of files first.
Dom knew better than to advertise his presence back in the criminal underground by using his own name. So he must have figured: what better alias than to let Eric take the blame for his crime spree...... This splainin doesn't take into account other characters played by Domenick Lombardozzi, like Detective Hauk on 'The Wire', or Officer Joey Kilmer in an episode of 'The Beat'. (Hauk could have been an identical cousin down in Baltimore, while Kilmer, who lived in the NYC area like Dom, might have been a half brother.)
'Entourage' - "Dominated", "Guys And Doll" & "Redomption"
'Bored To Death' - "The Gowanus Canal Has Gonnorhea"