It's not a speed racing, action filled, one bullet hunting the next one kind of movie but a very humorous one that focuses on the characters and the dialogues. And yes, Brendan Gleeson does a brilliant job!
The humour is british but not like in Hot Fuzz so it's not too weird and I don't see why people tend to say you got to have a sense of black humour.
The movie and it's feeling is in my opinion best compared to In Bruges. I really recommend this one.