Leonardo DiCaprio as BILLY is working undercoer for Police Officer QUEENAN played by Peter Mullan. QUEENAN's partner is DIGNAN (this most likely is Mark Wahlberg's role) and these two are the only members of the police force who know abut Billy's case.
COLIN (Matt Damon) is working for Irish Mob Boss COSTELLO (Jack Nicholson) who's had him infiltrate the Polcie Department to aid him out. At his side is MR. FRENCH (Ray Winstone) who's most likely his right hand man.