THE first thing you notice about Cillian Murphy’s new film Free Fire is the dirt. Essentially a shoot-out in a 1970s Boston warehouse, it’s probably the filthiest single-location movie ever made as characters duck, dive and crawl through the muck.

“I seem to be always covered in blood or some matter,” says the Irish actor. “On this one, they turned the sprinklers on me, so I was wet too. I’m always destroyed in some way in films it seems.”

Dressed in anonymous casuals, Murphy is sitting – freshly-showered and dirt-free it should be said – in a suite in London’s Corinthia Hotel. Amiable, down-to-earth company, on camera he’s a man of extremes. On Ron Howard’s nautical-themed In The Heart of the Sea, he starved himself on a 500-calorie-a-day diet consisting of an egg and salad leaves. For Peaky Blinders, the BBC series in which he plays 1920s gangster Tommy Shelby, he estimates he smoked about 3000 (herbal) cigarettes shooting the show.

In Free Fire, he plays Chris, an IRA soldier who arrives in Boston to broker a deal for a cache of weapons with a series of undesirables (led by Sharlto Copley’s South African arms trader). When an argument flares, shots are exchanged – and suddenly it becomes a battle for survival. A film that’s already been described as the “ultimate bar-fight” by Copley, Murphy readily agrees. “It is a lot about peacocking and masculinity,” he says. In other words, it deals with the idiocy of men who won’t back down.

This darkly comic gunfest is the brain child of Ben Wheatley, the English director behind cult films Kill List, Sightseers and A Field In England. “I basically pestered Ben Wheatley. I had seen Kill List in the cinema and I was absolutely floored by it. I thought it was the arrival of a significant voice in British cinema. We met and had beers and I was like, ‘Look man, whatever you’re doing, I’d like to be in it.’” Eventually, schedules aligned and Wheatley wrote the role for Murphy.

Dirt aside, the big issue was the guns on set (all shot in the abandoned Brighton Argus printing works). “I’m terrible to ask about guns – I can’t remember the names and I abhor them,” says Murphy. Real weapons were used, firing blanks; the noise was deafening, adding to the confusion, chaos and carnage. “We all had earplugs in, so sometimes you didn’t know what people would be saying! It was very tricky. That adds to the naturalistic nature of the performances – a lot of it was, ‘What the f**k is happening?’”

Last year marked Murphy’s 20th anniversary as an actor, though it wasn’t his first career choice. He was born in Cork, the eldest of four, and raised by his mother, who worked as a French teacher, and father, who was a civil servant. In his early years, music was his passion. “It’s something I did very seriously in my late teens and early twenties, and then I found acting, really.” His band, the Frank Zappa-influenced the Sons of Mr Green Genes, had a record deal on the table but they turned it down, feeling it wasn’t the right time.

Having failed his law exams at University College Cork in 1996, in that same year he was cast in the Enda Walsh play Disco Pigs. Originating the role on stage, this violent teenager was his first role of extremes – arguably setting the tone for his career – and it led to the 2001 film version, his first feature. A year later, he was working for Danny Boyle on zombie movie 28 Days Later. Other notable directors – Neil Jordan (Breakfast on Pluto), Chris Nolan (Batman Begins), Ken Loach (The Wind That Shakes The Barley) – soon followed.

In person, Murphy, 40, is the very definition of a ‘keep your head down’ actor. He’s been married to artist Yvonne McGuiness for almost 13 years. They recently moved back from London to Dublin, with their sons Malachy, 11, and Aran, 9. Despite his profile, Murphy is able to walk the streets without much recognition from fans and you won’t find him attending any premieres apart from his own. “I don’t get any hassle at all, which is lovely and the way I’d like to keep it. I’m very happy.”

Yet the big roles keep coming. Murphy’s enviable career continues with one of his bigger outings in the summer, when he appears in Christopher Nolan’s anticipated Second World War film Dunkirk. Like everything that Nolan does, there’s huge secrecy around the project – and it’s the only time in our interview that Murphy gets a little reticent, when I ask how his time was on the film. “Great,” he says, carefully. “It was…something.”

Still, after appearing as the Scarecrow in Nolan’s Batman trilogy and appearing in Inception, he must be one of the most called-upon members of Nolan’s repertory company. “I think Michael Caine is the number one,” he grins. “Sir Michael.” He’s quick to praise Nolan’s working methods. “The thing working with Chris is [that] it’s like working on the most expensive independent film you’ve ever been on…it’s a very small private environment.”

Right now, he’s filming the fourth series of Peaky Blinders, the show that has arguably cemented Murphy in the minds of audiences. It’s slated for one further season after this one. But could it go on? “Who knows? For me, I just want to end it on a high. You never want something to peter out. I was always very adamant – each season has to top the next. I have been disappointed by TV shows – there are laurels and there’s resting on them and we really wanted to avoid that.”

With Sally Potter’s acerbic comedy The Party also on the way – another one-location drama, like Free Fire – Murphy has also directed music videos. But largely he just wants to act. He’s not about promoting himself as a brand, like some stars. “It’s just about patience,” he says. “You have to be patient and hold fast to your principles and what interests you and what your tastes are, and are authentic to that. And if you’re authentic, people will see that and recognise it.”

He’s desperate to get back to the stage, though. “It’s high on my list of things to do,” he says. The last outing was Walsh’s play Ballyturk three years ago. “I feel if I leave it too long there’s a slight unbalance for me. I left it once for six years and things got way too unbalanced. I need to go back.”

It’s hard to imagine this very low-key man as anything but in a permanent state of equilibrium. “For me, you’re not really a performer unless you do live performances.”

Free Fire (18) opens on March 31