Killian Donnelly on The Commitments

And he’s delighted about it, well kind of.

“I’m playing a part that’s the most obnoxious, crude person ever, and the director Jamie Lloyd thought I was the man to do the job. Is that a compliment?“ asks the 29-year-old.

He needn’t worry as he is full of Gaelic charm as he tells me about how he came to land one of the hottest roles in the West End.

“I got a call from my agent saying they were doing a workshop for a show called The Commitments and it was about a band in Dublin. I stopped him and said “I know exactly what it is about, I know it off by heart”. He started to tell me what the character was about and I stopped him again and said: “You really don’t need to tell me. Just tell me where’s the audition?“”

After the audition he was overjoyed when the call came saying bosses wanted him for the role of Deco, made famous in the 1991 film version by teenager Andrew Strong (who was short, fat and ginger).

It had already taken more than two decades to get the project this far, author Roddy Doyle was none too keen on the idea of a musical for many years, and frustratingly silence fell for another two years, so Killian took leading roles in Singin’ in the Rain and Billy Elliot The Musical instead.

But earlier this year the phone rang again. The show was back on, with Roddy on board, and they still wanted Killian to star.

But it wasn’t a simple yes for him: “I had to make a decision whether to leave Billy and have an opportunity to audition, with no guaranteed role at the end of it, “ he says, “so it was a bit of a nerve-wracking situation for a while.“ As you may have already guessed, he did go to the audition and luckily that phone trilled one final time.

“I tried to be really cool about it,“ he says, “and was holding the phone to my ear and my agent said I had got the part and there was a pause while I turned the phone away and punched the air.“ Growing up in County Meath he says the book and subsequent film were both “iconic“, and when he told people back home they were buying him drinks and slapping his back.

“Straight away people were saying ’Are you top Deco or bottom Deco’ and ’but you’re not short, fat and ginger’. They all know the film so well.“ His character is the lead singer in a band of working class Dubliners who come together beautifully on stage to sing soul music, but suffer a clash of egos, and he says it is definitely the “biggest thing“ he has ever done.

No small claim for the man who has also starred in The Full Monty, West Side Story, The Phantom of the Opera and Les Miserables, and played the role of Courfeyrac in the 25th Anniversary Concert of Les Misérables at The O2 Arena and the role of Combeferre in the 2012 film.

“Yeah, I got called a veteran the other day,“ he laughs.

So how did he feel about taking on such an iconic role and singing massive hits such Try A Little Tenderness, In The Midnight Hour, Papa Was A Rolling Stone and, of course, Mustang Sally.

“I’m loving it. It’s completely different to anything I have done before and as it’s an original show you can’t do anything wrong, you are making it up as you go along in rehearsals.

“The number one thing was we didn’t want it to be a tribute act. Even the way I look is absolutely different from Andrew Strong, I have a shaved head and beard.“ And Roddy, who still lives on the street he wrote The Commitments on in 1986, was on hand to help him create his own take on the character.

“He was there every day at the workshop and rehearsals, he’s so down to earth, “ says Killian.

“The man is a genius and he’s written this brilliant piece and I kept going up to him and asking: “Was Deco based on someone?“ and he said: “No, I made him up.“ And I felt like a wanky actor then.

“But then I would ask if Deco would do this or that he would say yes or no straight away as he knows the character so well.“ The next challenge was getting the cast, many of whom are making their West End débuts, to come together as a credible band.

Killian says just like in the book and film at first they were plenty of mistakes, but now “we kind of have to pretend to be bad, as we are actually really good.“ And it seems his claim is no false boast as he has been receiving rave reviews for his role.

So what does the future hold for the south east Londoner, who spends most of his downtime playing computer games and drinking tea and says he is “so boring it’s ridiculous”?

“The happiest place for me is on stage in front of an audience,“ he says.

“I get nervous about a second before I walk on stage and the moment that leaves me is the day I will give up.“

• The Commitments, Palace Theatre, Shaftesbury Avenue. Details: 0844 874 0790, thecommitmentslondon.com