Excellent film. The great Ben Mendelsohn played the father, and I agree about his performance. Although I'm yet to see him turn in a bad showing in a film so far, he's very much the complete 'character' actor for me.
As for O'Connell, he's one of the brightest up and coming stars at the moment and is only going to get better imo (certainly come a long way since This is England). He has a really raw, natural talent and the sort of gravitas that is harder to find in the younger actors.
Recommend watching him in '71 if you haven't already seen it Chris.