Aside from the part where he gets shoved off a roof, we don't see much of Joey Doyle. We see Terry ask to meet him on the roof, and then—splat.
But, we get the impression that Joey was a really good guy. After all, he had the guts to snitch on Friendly's gang long before even Father Barry was in on the snitching action. And everyone thinks highly of Joey. His neighbor Mrs. Collins and a derelict longshoreman named Mott both say nice things about him: Mott says Joey was "a saint."
Joey becomes an example of goodness and innocence, unjustly crushed by evil. His death, according to Father Barry, is like Jesus' crucifixion that way. Even though it seems like a defeat, Joey's death actually gets his sister Edie, Father Barry, and Terry in a position to really go after Johnny and take down his mob.