I've never written a review before and don't really feel very qualified to do so, but I felt so strongly about this film that I wanted to do more to recommend it than giving the star rating.
Jim Sturgess turns in an incredibly moving and amazing performance as Martin, the young man who gets caught up with the IRA via his friends, only to be turned by "Fergus," played by Kingsley in a very different and understated role than we're used to seeing him.
Martin is torn between the cause and his friends vs. the ever-growing violence against innocents. He becomes a father and ultimately decides to be a source for Fergus, infiltrating deep and high into the organization. We live through his angst, fright, joy, sorrow, regret, rage and pride as he evolves.
Kingsley's portrayal of Fergus -- a hard and closed-off guy who comes to uncharacteristically care deeply about Martin -- is played brilliantly, with just the right low-key nuance in manner of speaking and facial expression that allow you to see his emotional wall crumbling a bit for Martin.
But there are costs for Martin regardless which path he takes, just a grim and sad result of the fractious climate between the IRA and British soldiers/police.
The storyline, the style of filming (sorry, I'm not adept with technical terms), the wonderful development of the Martin character (and to a lesser extent, Fergus), along with the incredible performance by Sturgess (I would go so far as to say even Oscar-worthy) really make this film memorable and worth your time.