Why didn't Delahunt stop Quinan from being...? SPOILER
If Delahunt was an undercover cop, why didn't he stop Quinan from being murdered? Delahunt was the only man with a shotgun so he could have stopped the others from killing Quinan, but not only did Delahunt not stop them, but he also removed Quinan's pistol.
There are possibilities I see with this. Delahunt having spent so much time with the gang, has developed some kind of attachment to them and has converted to their side, and is not a cop anymore in spirit.
Or, he never was an undercover cop, and the police lied about it to the press, in order to protect Costigan, so Costello wouldn't look for a rat anymore.
Could that be it?