One of Ireland's most internationally successful writers, Marita Conlon-McKenna - author of prize-winning children's novels such as Under The Hawthorn Tree, The Wildflower Girl, Fields of Home and The Blue Horse - sits in her bright suburban kitchen describing her lively terrier's recent brush with death following an operation.

Three of her four children, ranging in age from 11 to 21, wander in and out. The atmosphere is one of well ordered, if relaxed, normality which seems to suggest "Oh yes, Mum writes books." She makes tea, and the washing machine provides its own background music as she talks about one of her two new books, The Magdalen, her first aimed at adult readers. "When I was first asked why I wrote it, I just said, `I was adopted'."

At the time it was an instinctive response; now she knows why she said it. The story of the magdalen laundries - a generic name for mother and baby houses run by nuns - is one of the darker aspects of Irish life exposed over the past decade of our so-called new honesty. These women were, says Conlon-McKenna, "not wanted; they were pregnant, fallen, abandoned, in some cases mad and were just dumped in these places to get rid of them".

Established about 1830, these homes provided refuge for pregnant girls and women who were taken in to await the birth of their babies. Within weeks, having nursed their children, the women suffered the trauma of having to give their children up for adoption. The girls were treated as sinners and were expected to be grateful for having been given the chance to repent by working in the laundries. They, the Maggies, were referred to as penitents. "Don't forget," Conlon-McKenna says, "for the girls these places were also sanctuary." With an expression which conveys almost as much surprise as it does outrage, she adds, "I can't believe no one had written a novel about it before, or that no one seemed to know anything about them. How could this have happened?" Though not a particularly campaigning individual, she admits she is one of those people who "just gets so angry about things and then I'm off planning to write letters to the newspaper, whether it's about the Irish language, the traffic, dangerous drivers or the way we treat travellers". Letter-writing leads her to another subject. "I thought Christopher Nolan's new novel, The Banyan Tree, was one of most ignored books I've read. It's a beautiful story of a woman's life. I loved it. I never write to authors but I want to write to him."

One of the first things to grasp about Conlon-McKenna is that her mind works at the speed of light and digressions are frequent but seldom without a point.

When did she first hear of the magdalen laundries? "In the newspapers in 1993. One of the laundries was being demolished, the one in Drumcondra - all these graves of the girls who had worked there were discovered, and they were going to be disturbed. Suddenly there was a public outcry." When researching the subject she soon became aware of "so many gaps in the history". She also admits that when she began she thought the nuns would be monsters, "but this turned out not to be so".

She speaks at length; everything is well explained. Conlon-McKenna takes no short cuts. Her approach to the new novel is the same as for her children's books: direct and to the point. She does not believe in writing down to her readers. "I think books should be accessible. I write plainly, simply. That's just the way I write." Story is all to her. While she agrees that much of the relevance of The Magdalen lies in it being an important piece of social history, she also set out to make it one girl's story. "I didn't want it to be just about her time in the laundry. I was careful to give Esther a life before this happened to her." Almost 200 pages pass before she arrives at the magdalen.

She deliberately chose to give the young girl a romance with a handsome young blackguard who cheats her, rather than to portray her as a victim of incest, which many of these girls were - children had babies by their fathers, in some cases, repeatedly. Even before Esther is first devastated by the cruelty of her former lover, rejected by her mother and then forced to surrender her baby, she endures the drowning of her alcoholic fisherman father as well as the death of her six-year-old sister.

For all her hardship, however - and The Magdalen, set in the 1940s and 1950s, does not have a fairytale ending - Esther the Connemara girl, who is brought to a Dublin Magdalen to have her baby, chooses to stay in the city and make her own life. As her creator stresses, she is a survivor.

Did Conlon-McKenna think her own mother might have been in such a place? "I don't know. There's a chance. Maybe. I don't know anything about my birth mother. But at the time I was born, in 1956, girls were still having to give up their babies."

Conlon-McKenna grew up in Goatstown. Her younger sister, now living in Florida, was also adopted. Her adoptive mother, who died last April, was special. "To me she was Mum. I've never tried to track down my natural mother. I believe she is still alive. I hope she is well and happy and that she married and had other children, but I'm not searching for her; I'm really looking for me. For anyone who is adopted there is always this sense of a little piece of you that is missing. You are different. Several of my good women friends are adopted. We sort of found each other. When I met the editor of my book and she asked me why I wrote it, I said `Because I'm adopted'. And she said `So am I'."

At home nothing was ever said about her real mother. "I didn't know I was adopted until I was eight, and the neighbours told me," she says with a wry laugh. Did she ever wonder about her natural father? "Never." Surely she must feel some bitterness about the man who had put her mother in this situation in the first place and then failed to stand by her? "No, I've no feelings about him, No bitterness." All she has ever known about her background is that she was born in Dublin on November 5th, 1956.

School was close by. "I went to Mount Anville and I loved it. When we were first married, we lived near there for several years, until we moved here." Friendly, opinionated, interested in everything, a good talker, she makes surprising statements such as: "The problem with Irish is that it is compulsory. They should drop that and instead announce that only the 30 best students are going to be taught it. Then it would become interesting. There'd be a scramble. The mothers would be paying for grinds and everyone would want to learn Irish." She gives the impression of being capable and extremely practical, the definitive big sister type, possibly bossy, adept at sorting out even the most chaotic of situations in about 20 minutes.

In person she is a short, stout woman, someone whom you might turn to in the supermarket and who would agree the queue is impossible. The difference is that when she goes to the supermarket, children approach her looking for her autograph. Her voice is brisk, quite girlish and Dublin. But she is also slightly remote - forthcoming, though careful to keep her distance. It is not surprising when she says: "I can write anywhere. Here at the kitchen table, in the car, anywhere. My mind is always thinking of stories."

More than most writers, her writing has given her a sense of self. She seems confident of it ("I'm a writer, a natural writer, a born writer") while also remaining pleased at her success ("The books have done very well"). She mentions international awards she has won, including the International Reading Association Award, a US prize given to the best children's novel by a new writer and usually won by Americans.

Home for her is a comfortable, modern detached house in a quiet Stillorgan side road. The fact of being adopted appears to have determined much about her, even to using both her own and her husband's surnames. "My family are all important to me. When you are adopted you are so determined to make your own family." At 20 she met and married James McKenna. He is a banker and seems particularly easy-going, as he briefly half-listens to the conversation. Unimpressed at the fuss being made of the miraculously-recovered dog, he eats a sandwich and goes out. "He's not into dogs," she says.

They are planning a surprise party for their eldest daughter's 21st birthday later in the evening. "When I was 21, I was married and with my first baby in my own home," she says and adds, not for the first time, "no one ever realises," she corrects herself, "no one could realise the agony of giving up a baby until you have a child of your own. I think that's when what had happened to my mother when I was born finally hit me, when I was holding my own baby."

History has always been important to her, particularly as she has no history of her own. She speaks about her childhood and her love of books. "I was always reading. Children nowadays don't have the same level of reading as my generation - their vocabularies are smaller. The book is now under threat. I suppose that is why I do keep my language simple and try to draw the reader in at the start. I think it's good to let people know, particularly children, what they are in for."

School was wonderful she says. "I did very well. I was good at everything except for maths." But in order to go to university, she knew she had to secure a grant, which she did. "But my father had a stroke which he survived." She says of him that he was a remote father and proved a far better grandfather to her own children. But his illness meant she had to defer her place. "I had been going to do social science." Why? She seems such a likely history student, and history is an important element in her work; Under The Hawthorn Tree is set in Famine Ireland. "Oh I've always been interested in people and when I thought about it, that's what I wanted to do."

She never got the chance. Marriage and children soon took over. But she never lost interest in learning and was soon attending extra mural classes in a variety of subjects: from Anglo-Irish literature to women's studies, to creative writing, to children's literature. If there is a single influence on her aside from her own curiosity and drive, it has been Dr Pat Donlon, an authority on children's literature. "She has been wonderful to me and a real encouragement. I showed her some picture books I had made for my kids and she just got me going."

Donlon's support provided the deciding element; everything else was already in place. Conlon-McKenna speaks about her characters as if they are children for whom she is responsible. When researching the book which would become The Blue Horse, the story of a young traveller girl intent on getting an education, she met with several traveller families and visited their sites. It opened her eyes to discover how different levels of expectation are. "There was a young boy who wanted to be a dustbin man. That was a job he felt he could do. I met a young girl who wanted to be a nurse because she was good at looking after the other kids, her brothers and sisters, her mother, her granny, when they were ill. Not for a moment had she thought about the exams involved."

Later, while the women were having tea at her house, a service man came to fix the washing machine. "Having decided it was the home of settled travellers, he was very rude. James appeared down the stairs in his dressing gown and put him right." Conlon-McKenna's sense of justice is always on the alert, as is her sense of story.

Her new children's novel, In Deep Dark Wood, is pure fantasy, full of witches and mystery and belongs to the world of fairytale. While she is more apprehensive than she wants to admit about The Magdalen, which she says is "a strong story and I don't know how people will react to it", her face brightens when she describes the children's story.

Writing for children is the most important thing a writer can do as it does play a huge role in developing not just the readers, but the minds of tomorrow, she feels. At present her work room is a small, ordered place with a row of beanie babies on the high window ledge: a unicorn, a kangaroo, two snails and a friendly looking dragon. On the bookshelves is a small, blue wooden horse, a memento of The Blue Horse novel. Up to now, her books have been mainly set in Ireland, but she plans to do other things. This first adult book is another new departure. Her world seems very comfortable. Judging by the success of her books, is she rich? I ask half jokingly. "Sort of," she laughs before explaining writers don't really make that much money from books. "But we are comfortable; we have a nice life."

The Magdalen by Marita Conlon-McKenna is published by Bantam Books, £5.99 in UK. In Deep Dark Wood is published by O'Brien Press, £4.99