Did Queenan and Costello know each other since childhood?
I'm saying this because they both look to be approximately the same age (late 60'-ish to 70 years old) they both called one another by their first name, and both seemed to know each other's parents when Queenan made a comment about his mother and father and Costello made a comment about Queenan not having a father growing up.
It's possible both grew up to be on the opposite side of the law but knew one another pretty much all their lives. Costello also mentions Queenan when he first torture's Costigan when he tries to join his gang by banging his injured arm with a boot.