Dervla Kirwan: The actress is breaking hearts as a two-timing temptress in Pinter's Betrayal

Dervla Kirwan is getting used to adultery - professionally speaking. Her last stint on stage was at the National nine months ago in Exiles, in which she played a wife encouraged into infidelity by her writer husband. It was the first time that James Joyce's only play had been seen here since Harold Pinter directed it in 1970. Now she's preparing to appear in Pinter's masterful study of triangular desire, Betrayal, in which her character's seven-year affair with her husband's best friend is played out in reverse.

"It's just the natural law of things," she says. "Equally, I'd love to do a comedy. I don't want to be known as 'oh, Dervla, when she's on stage she's very good but she's terribly dreary', or 'another heavy night with Kirwan'." There is, nevertheless, an intensity about the 35-year-old actress, with her softly insistent Irish brogue and alarming habit of ending her statements with a widening of her piercing green eyes.

Betrayal sees her joined by the hefty theatrical triumvirate of director Roger Michell, Samuel West (who plays her cuckolded husband, Robert) and Toby Stephens (Jerry, her lover). "I walk into a room and I'm the only girl - which I really enjoy because it's like being with a bunch of brothers," she says.

They have taken a "forensic" approach to the script, creating an ample back-story for the characters. Matters are complicated further by the fact that Betrayal is based on real-life events - Pinter's seven-year affair with Joan Bakewell while he was married to Vivien Merchant.

"There really was a moral imperative to do the right thing," said Bakewell when the liaison was revealed in Michael Billington's 1996 biography. "I think it took Harold a long time to decide to come to terms with it by writing about it." Kirwan chooses not to get too bogged down in real life. "I'm not doing an impersonation of Joan Bakewell," she says. "I've really tried to capture that quintessential English quality of a certain class of woman who is just amazed at this wave of passion that has overtaken her life."

The cast have had lunch with Pinter and have just performed a first run-through for him - Kirwan seems rather taken. "He's got life in him, he's intensely charismatic and deeply attractive. He's a very sexy man."

Kirwan has her fair share of admirers too, being a household name for her television roles in Goodnight Sweetheart and Ballykissangel and, lately, a household voice for her honey-toned gastro-porn adverts for Marks and Spencer's food. She proudly informs me that sales of the chocolate sauce have gone up by 7,000 per cent, thanks to her breathy voiceover. "But could they send me a pot? No. They're extraordinarily tight. I'm very grateful for them supplementing two pregnancies and my career in theatre but would love a little bit of the love back."

For now, the television star is concentrating on theatre, partly because it's practical - being on stage allows her to live at home in London with her actor husband, Rupert Penry-Jones (Adam in BBC's Spooks) and their two children, Florence, three and Peter, one. "I've no desire to be away from my family for 16 hours a day," she says. "Really, right now this is perfect for me. If I could have my own little dressing room at the National, I'd be very pleased."

She is also desperately keen to prove herself. "I'm obviously slightly ill, because there is a burning desire to be perfect in me. It's probably the Catholic, or the ex-Catholic in me. Actually, Catholic," she corrects herself, laughing. "I'm trying to get my kids into the Catholic school around the corner.

"I now have to present a body of work and earn my stripes. In the past it was simply about putting a roof over my head and earning a wage. Now I really would like to slave away for a couple of years."

Kirwan grew up in Churchtown, Dublin, the third daughter in a "bubbly, loud, opinionated" family. Her mother taught English ("she'd have loved to have acted herself") and her father was an insurance executive. Unhappy at school - she still has recurring nightmares about waiting for her school report to arrive at home - at the age of 15 she was picked out by Charles Sturridge for a small role in the television drama Troubles. The next couple of years were spent as a jobbing actor, appearing at the Bush in London and the Peacock and Gate in Dublin by night and studying for her A-levels by day.

At 17, she left for London where she supported her burgeoning career with a reception job at the Groucho club. "I told them I was 21 and for about five weeks I was the worst receptionist ever. I thought 'asap' was a place," she recalls. She was, "thankfully", rejected from Rada following an audition with a "dangerously lascivious" teacher and gained a place at the Central School of Speech and Drama - which she promptly turned down. But Kirwan shrugs it off. "I come from a family of compulsive gamblers - my uncle, my mother and my sister gamble - so it's no surprise I chose something like this."

It paid off. Since her big break in 1991 with Melvyn Bragg's A Time to Dance, in which she played a schoolgirl who embarked on an affair with Ronald Pickup's ageing bank manager, Kirwan hasn't stopped working. Her television career rocketed with the two huge prime-time ratings winners Goodnight Sweetheart and Ballykissangel.

"I have a career because of that," she says. "The problem with Bally K is that it's 10 years old and, unfortunately, I haven't come anywhere near having that kind of success in a television role. The industry expects you to have hit after hit after hit and, if you don't, it's your fault. I guess I'm just having trouble coming up with my second album. Maybe there'll be a part that comes along that'll bring me back up there again. But maybe I've had my time. I've burned brightly and I'm happy."

She's keen to keep her "finger in that pie" and has just completed a six-part drama, True Dare Kiss, for the BBC. "It's great TV, but was such an unpleasant process," she says. "It was pretty soulless as an experience, a real kick, bollock and scramble."

Film doesn't hold much appeal either, despite a few close calls for big Hollywood comedies - "unfortunately Jennifer Aniston is always available," she says drily. "I used to think maybe I should go over there and get a series but now that I've got children, that's scuppered that. I'm not really too worried about missing that very slow boat to America."

In contrast to her onstage life, Kirwan is now blissfully settled at home, having lived out a couple of very high-profile romances in the past - with her leading man, Stephen Tompkinson, at the height of Ballykissangel fever, and the actor Darren Boyd. She met the dashing Penry-Jones when the two appeared on stage in Dangerous Corner in 2001.

"In the past I was competitive with an ex-partner and it was utterly destructive," she says. "If Rupert goes [to LA] - I'm not prepared to split up my family. I see what this business does. If it happens for him, I guess I will be the good woman behind the great man. There's no shame in that."

Does she still feel, as she stated in an interview 10 years ago, that she'd like to be working when she's 70? "Since I was 15 I've lived in fear that I'd never work again. Now, I'm 20 years on and I'm still living in fear," she says. "I'd like to be working, yes. But I'd also like to have about £15m in the bank."

'Betrayal', Donmar Warehouse, London WC2 (0870 060 6624), to 21 July