The Guard

Sergeant Gerry Boyle (Brendan Gleeson) along with FBI Agent Wendell Everett (Don Cheadle), ambush the drug smugglers at the docks, and shoot them all dead, including Clive Cornell.

Gerry gets on board the getaway boat as it heads off, however, a fire starts on board. He leaves Francis Sheehy-Skeffington to burn before seemingly going down with the ship.

It's implied he faked his death to avoid his corrupt past and start a new life, since he's an excellent swimmer.

Thanks ND Woods!