Gabriel Byrne is not one to mystify the 'craft' of acting. "We're all actors," he says, in a lush Irish lilt. "We spend most of our time acting. You act almost every day. You act different roles. The only difference is, I know I'm acting."

Having come to the profession at the relatively late age of 29, was it something of a revelation when he realised he could get paid for this subterfuge? "Er ... yeah," he grins, flashing a wolfish smile. "Overpaid sometimes."

He's too diplomatic to say, but he's probably talking about that era at the end of the Nineties, when the studios called and he played Satan opposite Arnold Schwarzenegger in End of Days and starred with Leonardo DiCaprio in The Man in the Iron Mask.

"I did two or three movies in Hollywood for money," he nods. "I had to make money. So I did a couple of movies that were box office things."

Largely, since making his debut in John Boorman's 1981 fantasy Excalibur, Byrne has avoided simply picking up the pay cheque, working with auteurs from the Coen Brothers (Miller's Crossing) to Bryan Singer (The Usual Suspects) and David Cronenberg (Spider).

More recently he won a Golden Globe for his role as a therapist in HBO drama In Treatment. He's 65 now but could pass for a decade younger.

While Hollywood and humility don't often go hand-in-hand, Byrne remains untainted.

"I still don't have confidence," he admits. "And I think what we project on to actors is delusional. We project on to people, who are human beings, this notion of an actor, that an actor is somehow somebody who can become other people. That's not what acting is. What acting is, is about courageously being yourself. That's what real acting is. False noses, wigs, limps, all that stuff - that's not acting!"

Raised Catholic in Dublin, the first of six children, the son of a cooper and a hospital nurse, Byrne has never had a problem with courageously being himself. A few years back, on an Irish chat-show, he revealed how he'd been sexually abused between the age of eight and 11 by Brothers in the Roman Catholic seminary when studying to be a priest. If that turned him against religion - "I'm what they call a recovering Catholic," he says - it took a long time before he found his true calling.

At University College Dublin, he studied archaeology and Celtic, and spent years teaching history and Spanish, before he joined an amateur theatre group. However, his reasons were more carnal than creative.

"I went into it to meet girls," he smiles. "I fell in love with a girl who was a banker by day and Ophelia by night. She had her hair blown back, and a huge, proud chest, and I thought, 'this is the life for me!'"

These days, he tends to work when he feels the need. His latest film, Louder Than Bombs is by Joachim Trier, the Norwegian director behind Reprise (2006) and Oslo, August 31st (2011).

"I had no idea I was going to work with him, but I realised from those films that he was a unique voice. A movie is the product of choices rejected and I understood all the possibilities that he had at his disposal. What he ended up with was a very singular vision."

Louder Than Bombs, Trier's first English-language drama, is the tale of a well-meaning father (Byrne) and his fractious relationship with his two sons (Jesse Eisenberg and Devin Druid), who are trying to cope with the loss of their mother (Isabelle Huppert) after a car accident.

It's a tender, tortured performance from Byrne, who has two children of his own, John 'Jack' Daniel and Romy Marion, now both in their mid-twenties, from his marriage to actress Ellen Barkin.

"I don't know many fathers who didn't make mistakes," he says. "I don't know many mothers who didn't make mistakes. It's all about making mistakes. And you're not necessarily rewarded for doing the right thing, or the wrong thing ... it's just the way life is."

It's why he loves the way the film.

"It doesn't end with an orchestra playing and everybody having a big group hug and an easy answer to life's complexities. Life goes on and there's no real reward for heroism, and the character I play is a hero because he gives out these little acts of kindness every day. I think being a parent trying to do the right thing is a very courageous journey."

Byrne divorced from Barkin in 1999, but remarried two years ago to his younger long-term partner, the US-born producer Hannah Beth King. Their age gap has never been an issue, he says.

"Not to sound too corny, but there is only one thing that matters in a relationship. You either love somebody or you don't. It's nothing to do with me trying to revitalise or have a second life or any of that bulls**t."

Like King, Byrne has enjoyed his own moments producing - most successfully on IRA drama In The Name of the Father. He''s next up north of the border in Mad To Be Normal, the story of Scottish psychiatrist RD Laing, to be played by David Tennant. But he's no more into looking forward than back.

"The past is intangible and irretrievable, as is the future," he says, enigmatically. So he's not nostalgic? "One day I woke up and said to myself, 'I don't have time anymore for nostalgia'."

• Louder Than Bombs is at cinemas now