Martin McGartland’s story is a riveting one. Where he was growing up in the 1970s and 1980s as a stone-throwing youth in the “Beirut” of nationalist west Belfast, the IRA were local heroes. He was not so sure. “Far from defending Catholics, it seemed that the IRA’s first duty was to wound, injure, maim and hurt Catholic teenagers,” McGartland wrote later, in response to the punishment squads that ran the estates. “The older I became, the more their actions angered me, driving me away from the republican cause.”

As a known petty crook and opportunist, running one step ahead of a kneecapping, McGartland was easily tapped up by the security services for info on suspected recruits. Later, after a personal epiphany over the Enniskillen bombing in which he rejected the notion of the armed struggle, he turned all-out informer. Infiltrating the IRA, McGartland worked from the inside — a traitor to his kind, a reviled “tout”, facing a hell on earth if exposed. To the Special Branch, meanwhile, he was “Agent Carol”, one of the most valuable spies on its payroll, his intelligence used to thwart numerous paramilitary operations. Inevitably, his mission would be compromised. In 1991, after two years embedded, McGartland was captured. Facing execution, he miraculously escaped and began a new life, under an assumed identity, in England.

McGartland wrote about his experiences in his 1997 autobiography, Fifty Dead Men Walking — the title a reference to the number of lives he is attributed with having saved. And now comes the film version. An Anglo-Canadian production, it’s a powerful ticking-clock thriller, a tale of life on a knife edge, of a man conflicted over loyalty to his community and the compulsion to do the right thing. Of note, it features a remarkable performance by Jim Sturgess as McGartland, clinging to his sole fraying lifeline: the one to his handler (Sir Ben Kingsley), whom he can contact only through the arcane means of calls from public phone boxes.

For the soft-spoken actor from Surrey, it is a transformative turn, far removed from his two significant films to date: Across the Universe, Julie Taymor’s ambitious love-it-or-hate-it Beatles musical; and last year’s 21, about a bunch of MIT students scamming the blackjack tables in Vegas. Neither film had come out at the time of casting, he reminds me. “It was a mad period. I was getting things without anything being seen.”

Forced into a snap decision on this film, he was “whisked off to Belfast pretty quickly”, he recalls. There, he channelled himself into the persona of the ducker and diver, the seller of knock-off gear, whom we first meet when he is aged only 17, but whose spark remains irrepressible, despite his increasingly grim circumstances. According to locals, Sturgess’s accent is spot-on, honed from living for weeks in the community where the film is set. “I made a decision to stick with the accent for the duration, which helped. It did bring you into the character in everyday life.” He laughs. “It gives you a golden ticket to be really cheeky. It has a real playfulness.”

Beneath the superficial swagger lay a deadly serious mission, of course. For, as shown in the film, McGartland’s story did not end on the streets of Belfast. In 1999, he was tracked down by a hit squad — allegedly IRA — to Whitley Bay, Tyneside, and shot six times at point-blank range. Miraculously, he survived and returned to his life in hiding.

Northern Ireland’s tragedy continues. Indeed, Fifty Dead Men Walking is opening in very different circumstances from the ones in which it was filmed. Until recently, it appeared but the latest entry in a string of flicks about the Troubles, like the Hunger, made during the comparative comfort of the Paisley/McGuinness governance, when sectarian atrocities were assumed to be confined to the rear-view mirror. Then, alas, came the latest dissident killings.

“Having given careful consideration to the recent events in Northern Ireland and following consultation with the relevant parties, Metrodome has decided to continue as planned with the theatrical release,” comes the statement from the film’s distributor. “The general consensus was that plans should not be altered. However, we are monitoring the news daily and will keep this decision under review.” Set to close the Belfast film festival on April 4, the film may yet receive a bumpy ride, not least because in re-creating its scenes of paramilitary cell activity, it employed former IRA men as technical advisers.

“We were introduced to people from all different sides, very much so the people from the IRA,” Sturgess confirms. “We were introduced to their lives and were vouched for by these guys. We were taken to pubs around Belfast to which we would never have dreamt of going had they not been with us. It was confusing because you got really drawn by that . . . ‘Oh, these are family men.’ Then you step back and go, ‘No, they are also capable of being incredibly violent.’ There was one guy I spent most of my time with, and we got very close. And this guy, who was so nice, had been in prison for 15 years, and you realise he had done some horrific things in the name of what he believed, his cause.”

And that specific on-set advice? “Without glamorising the world, they just wanted to tell us how it all went down and how they would have approached things,” he says. “When we were doing scenes like making a bomb, they would say, ‘There’d be someone over there. You wouldn’t have put on your balaclava yet.’ Just all these little things to make sure it was authentic.”

Sturgess must choose his words carefully. Recently, he got burnt after an interview in Empire magazine was seized on by The Belfast Telegraph under the headline “IRA were some of the nicest people that I ever met, says actor”, provoking an understandable howl of outrage. He is not the only one to have landed in hot water. Last September, as the film was unveiled at the Toronto film festival, his American co-star, Rose McGowan, who plays a senior provo, claimed: “I imagine, had I grown up in Belfast, I would 100% have been in the IRA . . . My heart just broke for the cause” — a statement more in keeping with the misty-eyed, fiddly-dee sentiments expressed any time a big-shot Hollywood actor takes up the prop-shop Armalite. “It was a shame that she said it,” Sturgess says. “It was insensitive and didn’t come from a particularly well-founded base of knowledge.”

That Sturgess got here, he confesses, is still something of a surprise. Not long ago, he was toiling away on television in Heartbeat and The Quest, with David Jason. The 27-year-old, who looks like a cross between a young De Niro and (appropriately) a late-Fabs McCartney, had chosen a media course at Salford University, “to see if I could join a band”, until a self-penned one-man show got him a theatrical agent.

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