The elder McDonagh has a slightly mechanical gimmick to inspire his screenwriting: he takes all the clichés in American detective dramas and has his characters do the exact opposite. Thus, he’s made a message movie about prejudice and xenophobia: namely, they add a bit of fun to life! In The Guard, the rural Irish resent the big-city Dubliners, all the Irish resent the English, and everybody in the British Isles resents the cultural dominance of American crime shows and movies.
The movie is set in County Galway, where something like 10% of the people still speak Gaelic. Driving through Galway in 1987, I asked an old man for directions, but he didn't speak English. That happens to Don Cheadle in this movie, but he finds out later from Brendan Gleeson that they were just saying in Gaelic, "If you want to speak English, go to England."
It would be cool to have your own secret language.