"The whole nonsense that surrounds the industry, which can be very trying to any sane person wanting to have a normal life. I used to find that very wearing."
Firmly an actor's actor, it's unlikely you'll catch Murphy, who turned 40 in May, ducking in for a selfie with his Dunkirk co-star, Harry Styles, any day soon. Dunkirk is the new Christopher Nolan war film the pair star in which is due out next year.
"It's good to have ambition and hunger and all that, but it's not about being the best, or having the most money or the biggest trailer," he reasons. "It's about trying to really work hard."
Born in Cork, and originally a law student before the lure of playing in a band and ultimately acting came knocking, solid, fulfilling work has always been Murphy's professional guiding star, and not the Hollywood lifestyle that comes with it.
And since his stirring performance in Danny Boyle's 28 Days Later back in 2002, he's more than proved his mettle, immersing himself in roles as diverse as the unnerving Scarecrow in Batman Begins, a passionate IRA freedom fighter in Ken Loach's The Wind That Shakes The Barley and, of course, as intimidating Tommy Shelby in BBC hit Peaky Blinders which has seen the Irish star in the region filming frequently at locations such as the Black Country Living Museum.
After a long spell in London, he's now settled back in Ireland with his artist wife, and their two sons and black Labrador puppy.
He says he's constantly asked about career plans – much like the gritty and violent Tommy in the Birmingham, Black Country and London-based gangland drama.
"People always go, 'tell us your strategy'. That's for economists. We don't have a clue what's happening next," says Murphy, looking stylish in a plain button-down blue shirt and jeans, his hair grown back from Tommy Shelby's brutal do. "Genuinely, the whole thing has been chaotic, but you obviously have things you want to do and things you know you won't do – and then life gets in the way and the availability gets in the way. It's entirely random," he says of his own path.
"Maybe some superstars have it all mapped out, but I've never met one that does."
Unpredictability though, is part and parcel of acting.
"It's joyous and terrifying and frustrating, but when it works, it's great," he says.
"In terms of work and the people you work with, you want to try and better that. Not knowing if you're going to be able to sustain that level, I worry about that. But you can't control it."
What you can control, he says, is the work you say yes to.
"I'm more choosy about the stuff I do," he admits of getting a little older.
"If I'm going to go away from home and leave the family, it's got to be worthwhile, so that is different than when you're a naive, hungry 20-year-old."
Anthropoid, his next film, more than ticks these boxes.
The thriller tells the little-known but true story of a group of men who lived in occupied Czechoslovakia and were involved in the assassination of Reinhard Heydrich, Hitler's third-in-command – and the main architect behind the final solution – whose regime of terror earned him the nickname, the 'Butcher of Prague'.
Knowing Heydrich's death would be a huge blow to the Germans, the men need to act quickly, all the while knowing any show of resistance would have deadly consequences for them and their loved ones.
Alongside The Fall and Fifty Shades Of Grey star Jamie Dornan, who plays Jan Kubis, Murphy stars as Slovak Josef Gabcik, a fellow soldier from the Czech Army in exile, who is parachuted with Kubis into Czechoslovakia in 1941.
From the outset, the actor was intrigued.
"This film is slightly unconventional or old-fashioned in its storytelling, and that's something I'm drawn towards," he explains. "You get to spend time with the characters before the next set-piece happens. You're actually invested in them. War films work when you can see yourself in that position, and you say to your partner or whoever you're going to see it with, 'what would you do if you were in that situation?'
"With these guys, you go, 'they're terrified'. Josef tries to put on a front and make himself the most practical and determined version of himself he can be, but underneath it all, he cannot think beyond the mission, so I hope that it makes it more identifiable and relatable."
Directed by Sean Ellis, the film doesn't shy away from depicting the brutality and torture that featured in Heydrich's regime, something Murphy believes is necessary.
"I hope and I believe that this film doesn't in any way glorify or make it gratuitous in any way, but it's important to show that these people lived besides this all the time," he says.
"The country was occupied and these people went around in fear of their lives, so when you see the retributions carried out, you can really feel the guilt that these men carried around. That's why it's useful to show the dilemma these men faced before they carried out the assassination and after it."
It takes a thoughtful persona to carry a film like this, and Ellis was keen to secure Murphy from the outset, dubbing him 'one of the finest actors of his generation'.
"It's very kind of him to say that and, you know, it was well worth investing in dinner last night, clearly," he says with a laugh.
"You keep on working and try and do the best you can. That's really the only motivation," he adds.
"Legacy and any of the other stuff is for other people to talk about, I just want to do the work as best as I can."
l Anthropoid is released in cinemas tomorrow, with the fourth series of Peaky Blinders expected later this year or early in 2017