I see that this film has been getting great reviews 3 things that ruined the film for me.
1 - The film is supposed to be spread over a fairly long period of time BUT there is no indication of this as you are watching it, this can become confusing at certain points.
2 - If someone in your gang is a snitch / mole and you're a super clever gang leader surely you can work out that it's the guy who you know "dropped out of the police force".
3 - If there is a mole in your elite police crime unit, who's sole job is to nail the leader of an Irish gang. It doesn't take that much common sense to work out its going to be the guy called "Paddy McFinnigan" (can't remember his real name but its that obviously Irish).
I would best describe this film as a lame duck version of Donnie Brasco.