So I was reading this list of the "Top 150 Guy Movies of All Time", putting the ones I hadn't seen into the Blockbuster rental queue. This one finally showed up.
It's about two nice Irish Catholic lads from Southie who, for hazy reasons, get into the game of vigilantism. Willem Dafoe plays a brilliant gay FBI investigator hot on their trail. The dialog is funny, the action is very violent. While it wasn't a critical success (20% on the Tomatometer), its unique premise, offbeat characters, and unusual style worked for me.
The reliable IMDB trivia page counts 33 bodies, and 246 occurrences of the f-word. Frankly, I would have guessed higher than that latter number; in a 110-minute movie, that's only 2.24 occurrences per minute! Seemed like more.