The Down-born writer Eoin McNamee brilliantly described the Northern Ireland peace process as "like sitting in the dark watching a movie for 30 years and all of a sudden somebody turned the lights on and stopped the projector and you realised that all the action had been going on in the projection room all along". It's an analogy that comes to mind watching Shadow Dancer, Oscar-winning Man on Wire director James Marsh's twist-filled film set during the final days of The Troubles.

Adapted by ITV News Political Editor Tom Bradby from his book of the same name, Shadow Dancer follows IRA member Colette McVeigh (Riseborough), who is apprehended by British intelligence services in London and given a choice: become an informer or spend the next 25 years in prison while her son grows up. The single mother has risen high on the radar because her brother Gerry (Gillen) is one of the leaders of the IRA in Belfast and is seen as one of the last stumbling blocks to a viable and lasting peace. Taking the British offer, she returns home with details of how to meet with Mac (Owen), the London interrogator who has now become her handler. He guarantees her safety, but he too is a pawn in a bigger game.

While Marsh is best known by audiences for his documentaries (he also made the Oscar-nominated Project Nim), Shadow Dancer proves his gifts also extend to the thriller genre. Recalling paranoia-cranking gems like Defence of the Realm and The Parallax View and 1980s TV series like the BBC's Edge of Darkness, it's a movie that keeps you guessing and has some standout bite-your-nails-to-the-quick scenes, including an early sequence in London and an IRA mission in Belfast (the movie was shot in Dublin). From frozen-in-time kitchens to drab offices to backstreet bars, the smog of doom hangs in the air and Marsh brilliantly instils the feeling that the walls are closing in on the characters - no matter which side they're on.

In the lead role, Riseborough shows once again that she can be among the most acclaimed actors of the coming decades, with Owen also giving one of his finest turns as the war-weary agent who also gets stuck in the middle. A great Irish supporting cast add much to the grittiness, with the fact that Bríd Brennan actually looks like on-screen daughter Riseborough one of many little things that add to the film's sense of authenticity.

You may have a headful of unanswered questions at the end, but they only add to the allure.

Harry Guerin