Cecelia Ahern says her début novel is the product of her imagination, not her life as the Taoiseach's daughter. And the money is nice, she tells Kathy Sheridan

OK, let's cut to the chase. Your first 10 chapters charm a UK publisher into parting with £150,000 (more than €200,000) for a two-book package. In a separate deal publishing house Hyperion lays out $1 million (about €800,000) for the US rights. Meanwhile, publishers from 22 countries around the world are ripping out their chequebooks to grab the rights for their territories. Then one day you're out shopping, excavate the mobile from your bag and, hey, it's Wendy Finerman, the Hollywood producer who made Forrest Gump. She just wants to congratulate you on the book and mention that her screenwriter might be in touch for a dig out with characterisation. That's another $100,000 up front for the film rights - oh, and $500,000 more if they make it to the first day of filming.

Of course, no 22-year-old first-time novelist would get away with a plot like that. In fact, plenty of people say no 22-year-old should get away with that kind of thing in real life. It stands to reason that it's only because her father is Bertie Ahern. As theories go, it makes little sense. A nanosecond's consideration of a typical chick-lit reader in Tennessee or Thailand, never mind a hard-nosed Hollywood producer (whose screenwriter, Steven Rogers, is into his second draft and hasn't troubled the author so far), should quash it.

Cecelia Ahern shrugs in the manner of someone who has heard it all. "I'm really proud of Dad, and it's great to be linked to him, and I can understand that in marketing you need a whole new angle, but it's an insult to the publishing world to say that they would buy something simply because of that."

What must it be like for her father to have two daughters who are richer than him? (Ahern's sister, Georgina, is married to Nicky Byrne of Westlife.) "Poor Dad," she laughs. "And he works harder than the two of us put together. But politics is his absolute passion and he's reached the job, and I can understand . . . . It makes him happy. He does it 24/7. With Dad, for the amount of time he puts into it, it has to be the passion."

It is immediately clear when she is uncomfortable with a topic; her face flushes bright pink, to match her loose-fitting jumper. Sitting in a comfortable bedroom in the Merrion Hotel in Dublin, she has presented herself for interview without a minder or tape recorder, with minimal make-up and looking not a day over 16.

She signs the chit for the coffee rather awkwardly, betraying nothing of the flash, been-there-done-that persona one might expect of someone who has already been fêted around US booksellers, hitting four states a day, and who has been interviewed, flattered and photographed for everything from USA Today, Vogue and Marie Claire to Time and People magazine, as well as sharing the spotlight with nine other "rising artistic talents" in the Observer magazine's edition on the best and brightest of 2004.

She is persuasive in her repeated insistence that she is a "shy" and "very solitary" person. The odd thing is that this woman who grew up in the media shadow of a high-profile father chose to study journalism and communications, of all things, at Griffith College, Dublin. "What I've learned is that I crave privacy. It's the kind of person I am. I think privacy is the most sacred thing ever, and once you've lost it you've lost something very special. I have such a private life. I just want to talk about my book. And it is just a book. I'd really be at home if it was not for this book."

And although she might flush bright pink while refusing to comment on unwelcome topics, she is resolute. "I'm well able to say 'no comment' when I don't want to talk about something. I've grown up reading awful things about Dad, and, yeah, things do sting a bit, but it wears off. I just feel sorry for Dad. He is such a nice man, it's horrible." She is equally adamant, however, that people cannot be dragged into the limelight by the media unless they choose to be. "My mother stayed out of it. I think you can walk into it . . . . It's completely by choice."

Her words are borne out by her actions. She ignored the old first-time- author dictum that you write about what you know. "It's called imagination," she says brightly. "I was very conscious that people would be looking for autobiographical stuff. I went out of my way to make this different [from my life\], and I was showing it to my mother and family members all the way through. You don't put family members in a book - and Holly [the central character\] is nothing like me. It's not a diary, it's fiction."

So how did she garner her insights into, say, bereavement? "I don't know anyone who's lost anyone that I'd be even close enough to to talk about it. I'm alone a lot. I think a lot. I question myself a lot. I went into the depths of my imagination. You look at people around you and you learn." The large family scenes? "That's just fiction." The incredibly united, calm, supportive parents? A tad unrealistic, perhaps? "It's not about them. It's about Holly, and they are among the people who were there for her."

We tease this out and she holds her ground. "I see older people all the time holding hands . . . don't you?" But is this marriage not somewhat idealised? "I don't go into their . . . into their . . . . It was about Holly, and they were there for her and that was their role in her life. If it was a book about her parents, then perhaps I could go into things that happened over the years, problems in their own marriage; there could be a story there. But, generally, I think there are some people who have happy marriages. I think in some cases it can work. I think in a lot of cases it just can't. People change, people grow apart, and it does happen. And you can fall out of love just as easily as you fall in love, so . . . it happens."

She also defends her rather featureless portrayal of 21st-century Dublin. "There is nowhere recognisable in there because it puts a time on a book, and I want this to be timeless . . . . It was never important where Holly was." How about the strange but true fact that it is a chick-lit novel without sex? "I didn't think it was important for the book - and I would have been really embarrassed writing it."

Ahern's confidence is high, so high that it will take something unusual to buckle her. She has scanned the Irish reviews so far and adjudged one to be "not a review at all", while the others, though by no stretch ecstatic, were inoffensive, with one even somewhat favourable. "I'm under no illusion whatever that I'm the only author who's going to be loved by the world. If there is constructive criticism I'm going to take it if I agree with it," she says modestly before adding cheerfully: "So far there's nothing I agree with."

So there is nothing she would have done differently with the book? "No, there's nothing I feel I could have done better. Nobody can touch me on it. I'm delighted with it . . . really, really proud."

She has always written stories, she says, recalling that as a 14-year-old she tried to write a book in which the central character was 16. So money was "definitely, definitely" never the driver. "I think this book is written so well because it's written completely from the heart. All this" - the money and so on - "comes to people who don't go looking for it."

And it's still not about the money, she insists. "It's not important to me. I'm not a flash person at all. But it's an incredible bonus. It means I can help out friends and family, and it has allowed me to move out and get my own place, an apartment." She would like to live beside the sea. "I love the seaside, but prices are crazy." But hasn't she wads of money now to indulge that? "People keep reminding me of that, and it's true. But I'm such a careful person. I don't splurge out."

She works when others of her age are out clubbing. She wrote PS, I Love You in three months, in the family dining room, to the exclusion of sunlight, friends and family, writing in longhand from about 10 p.m. until 6 a.m. or 7 a.m., typing it up the next day.

She was also lucky to have two indulgent parents, at that juncture between getting her degree and starting a master's in film studies, when the gift of time left her free to pursue her goal. And her run of luck culminated in finding an agent of the calibre of Marianne Gunn O'Connor.

For a 22-year-old she seems solidly settled. Her drink of choice, she claims, is 7 Up. "I get too hungover. It's not worth it." Her boyfriend of nearly three years, David Keoghan, is an athlete and shares her restraint. "It'd be one or two drinks every few weeks." And he is the love of her life. "He's the one. I can see forever with him. I have no plans to be leaving him any time in the next 80 years."

PS, I Love You is published by HarperCollins, £10.99 in UK