In this case that actor is Brendan Gleeson. Every other person in this film could be replaced by a long list of actors--and some of them are fine actors--and it wouldn't matter a whit. Gleeson, by the very nature of this film has to load it on his shoulders and carry it. And he does.
Usually when one actor has to carry a film it doesn't bode well for a movie. Think of all the Robin William's vehicles of years past...brr...brrrrr...BRRRRRR. I am going to need a couple of shots now. Sure, Williams, has done some good work but when a movie was laid on him? What do you wind up with? Good Morning Vietnam or that one where he plays a robot.
There are other examples of this and it usually means, at best, a mediocre movie. In this case all that surrounds Gleeson IS good and the dialog is unrelentingly witty. Often the lead actor is the key to a film but a film can still be good with them being mediocre. Any number of Ben Affleck films bear this out.
This is a very funny film. That has to be said and should have been said earlier. It is a comedy and rare are the actors who can be funny like this. Few are the actors who can pull of a role like this.
It has hints of films such as Snatch and also other "rural" Irish comedies. But, it never gets "precious." Go to see it, it will likely be out of theaters soon. If you miss it there be sure to put it in your Netflix cue.