This film serves a great purpose: whenever someone tells me that they love Boondock Saints, I feel justified in ignoring everything else they have to say about cinema. It is the worst movie I have ever seen. The only people that have ever said anything positive about this movie have been highschoolers that don't know anything about cinema.
This movie isn't entertaining to me, but that's a subjective claim. This movie also happens to be completely unoriginal. It's about vigilante justice (70s crime movies anyone? Dirty Harry? French Connection? They all do it better.). It's about Irish-Catholic violence in Boston (even though it came afterwards, Gangs of New York portrays this theme more powerfully). It has characters marked with meaningful tattooes (Belly, Memento). It includes a cross-dressing detective (David Duchovny in Twin Peaks? What was that, 15 years earlier?). It depicts prayers before murder (Godfather Trilogy, Pulp Fiction). This movie was so unremarkable and derivative that I can't even remember more things to criticize.
On a final note, it's just not a cool movie. The things that the filmmakers and the appreciative audiences think are cool are just not. The violence is sluggishly portrayed. The gay detective is just a caricature of an emphatic queen (unlike Gay Perry in Kiss Kiss Bang Bang). The whole movie sucks. There is nothing to like about this that hasn't been done a million times better in other films.