Ben Mendelsohn is a bit young, so I presume the source has been rejigged
He is miscast. The lead would ideally be played by someone of Gary Oldman's or Stanley Tucci's vintage. Roger Allam was in his early 50s when he debuted the part. Ben Mendelsohn plays seedy these days, but he's 10 years too young and he additionally has an energetic quality.
In the play, Ray confides to Una that he thinks he will be dead at 60, and he only has 4 or 5 years. Given that the play was (effectively) a two hander it required restructuring for the screen, I suppose some of his scenes will be set before he and Una reunite and they will age him up for the bulk of the action.
Times are bad. Children no longer obey their parents and everyone is writing a book - Cicero