The film is loosely based on Martin McGartland's autobiography covering his time as a tout informing on the IRA during the Irish Troubles. Although it has been criticised (by McGartland in particular) as not being an entirely accurate portrayal of the events that happened it does actually clearly say that it's based on those events, but has been changed in both the opening and end credits.
Even given that, it still gives a very moving portrayal of the Irish conflict and you get a strong feeling of sympathy for the McGartland character, given it portrays him in a dangerous situation throughout most of the film. It gains your sympathy from the opening scene where you see the 1999 attempt on his life and it leaves him dying in the car as it goes back to 1987 to the main part of the story and doesn't resolve the opening scene until the end.
The film is a very uncomfortable watch in places, particularly the scenes showing the IRA interrogating the touts. However, it does demonstrate how much of a tragedy the whole conflict was and shows that the blame lies equally with all sides involved and you're left with mixture of sympathy for all those trying to promote their causes and horror for the way they're doing it.
There are really strong performances from a lot of the cast, but Jim Sturgess puts McGartland's character across with amazing conviction and you get a strong feeling of the fear that must be going through his head towards the end.
Not an easy film to watch, but worth seeing, just prepare to be shaken up by it.