Eric lead the Irish Gang to Vito
It's know in some material of the game that Eric was part of a small time gang. This video on Vito's life
posits that Eric Reilly was part of Brian O'Neill's Irish gang, and I think this makes sense.
Picture it: You're part of O'Neill's gang and in Hartmann Penitentiary. You've just heard word that Brian has been murdered, and no-one can figure out who did it. Well, the solution seems obvious; there's one Italian inmate Brian kept going on about, blaming him for getting locked up. He got into a big fight with that same guy in the yard just a couple weeks ago, and now all of a sudden, he's dead. It seems cut and dry, so the solution is to bump that inmate off. The problem is, he's been connected to some high-ranking mobster and you can't touch him. Then before you know it, that same guy gets out early, and he could be anywhere.
Now, one of your own shows up with the shit kicked out of him. He explains that his brother-in-law did it. When he mentions who that brother-in-law is, you've finally found the guy who killed your boss. As the brother-in-law, surely it makes sense that he would know where the guy lives...
