`I'm not by nature a farmer's wife. I don't like sheep and I hate cows. I had to milk the rotten cows." Late afternoon sun pours in through the Georgian windows of Christabel Bielenberg's home near Tullow. Her husband Peter goes to see about the harvesting while she reminisces about how she began to write her famous memoir about surviving in Nazi Germany, The Past is Myself (first published in 1968, now printed in seven languages and on the school curriculum in Ireland, England and Germany). "We came to Ireland in 1948. All our friends in Germany had been murdered by Hitler so we left. I couldn't expect my German husband to live in England, which was still very anti-German. So we came to Ireland. My father was Irish, from Corofin, and I had always loved Ireland. I used to come here for school holidays.

"Luckily for us land prices then were absurdly low. My first years here were very strange. The house was in a pretty bad state, and the farm had been let go to wrack and ruin. Peter had been a lawyer in Germany; he was only a beginner as a farmer. He had a series of books, published by OUP, called Teach Yourself Farming. I disappeared into my writing." She smokes, gesticulates, laughs often and heartily, and has a charming girlish smile. It's hard to believe she's 90. And we have shared a pot of tea before I realise she's nearly blind.

When she first decided to write a book, she thought she would tell the story of her ancestors. "My lot landed in Clare in 1651, two farmers from Shropshire: Francis and Thomas Burton. My father always used to emphasise to me that we paid for our land. It wasn't a Cromwell thing." But she got stuck because "my ancestors were too dull and worthy. They didn't misbehave at all. They were boring and far too pleased with themselves. So I decided to write my own story. The German thing. It was gorgeous when it went well. I got lost in it."

She had her first taste of the joys of writing just after the war, when she was employed as a war correspondent by the Observer. "Peter was still in Germany. The Britishborn wives got out but not the German husbands. I was friends with David Astor (editor of the Observer) and asked him how I could get Peter out. He sent me to Germany as a war correspondent. I spoke German fluently which was a huge advantage. It meant I could do more interviewing than most of the other foreign journalists." Being "a resourceful nose-poker into other peoples' affairs" also helped in the writing process.

Her ringing Anglo voice is ageless: she enunciates her words and projects them across the room in a powerful, articulate flow. "I like an audience," she confesses. "I only ever got stuck for words once. I was giving a talk to a lot of bone-faced people in Germany." She laughs and lights another cigarette. "My voice was trained for singing. The smoking hasn't ruined it yet."

Her first ambition was to take up the scholarship she won to Oxford to study English, but she changed her mind. Influenced by her father and his talent for singing, she decided to become "the world's most powerful dramatic soprano". She has fond memories of singing with John McCormack (whom she met through her father) and his daughter Gwenny in Moore Abbey, Co Kildare. She went to Hamburg to study music and met Peter there. "John McCormack gave me a terrible dressing down for getting engaged to Peter. I felt I could go on with my training after I got married but I didn't."

She has no regrets about having made this choice. After 64 years of marriage, she and Peter are obviously still very close: "We've had a varied and interesting life; we've solved many things together." Her face melts: "Peter was a beautiful dancer. With him, I realised what dancing was all about. I think if I'm really honest, that's when I fell in love with him. When we danced together, people left the dance floor and stood around watching us."

Peter, a tall good-looking man the same age as his wife, brings us our tea. Although Christabel encourages him to get involved in the conversation, he retreats quietly after depositing the tray, leaving his redoubtable wife to continue talking. When I ask her which part of The Past is Myself was the most painful to write, she recalls the four weeks after the failed attempt on Hitler's life on July 20th 1944, when she didn't know whether Peter (who had been involved in planning the bomb plot) was alive or dead: "I heard nothing until I finally got a letter from a friend saying he'd been arrested." Then there was the horrific news that their close friend, Adam von Trott zu Solz, who was also involved, had been hanged. Bielenberg made haste to visit her husband's former boss to ask for whatever help he could give, promising that when the war was over she would do what she could for him.

Even more riveting is her account of visiting Peter in the concentration camp where he was detained, when he managed to pass her a matchbox with a tiny message (which she slipped undetected inside her glove): "They allowed me to see him, because I think the Gestapo knew the war was coming to an end. I was extremely lucky not to have been searched."

She then volunteered to be questioned by the Gestapo, hoping it might speed up Peter's release. Having received Peter's message, she knew what he had told his captors, and was ready to tell the same story, knowing it would tally with his. Her fear was transformed to fury when, waiting for the interrogation, she saw an elderly man in chains being beaten across the face by a Nazi secretary: "The man dictated a letter to her, and when she had finished typing, he started to read it. She was very impatient and said: `Do you think I have not written it down properly?' and he replied `I wouldn't know'. She then proceeded to smack his face right and left with a vicious look on her face.

"I've never been so angry, and do you know, it's impossible to contain two violent emotions at once, so I wasn't scared anymore, I was livid. I was called in to the interrogation and told to sit down. There were lights shining in my face and I couldn't see a thing. I was still very angry, so I shouted: `Turn off those lights, how do you expect me to see the chair?' And the man did turn off the lights, which made the whole thing much easier."

She used every tactic her lively brain could conjure to try and convince her interrogater of Peter's innocence: "The Gestapo fellow, Kriminalrat Lange, said `your husband is a political idiot' and I said `how clever of you to have noticed'. I promised Lange my relatives would help him after the war. I gave him every kind of surety." She mentioned her uncle, Lord Beaverbrook, then Minister of Aircraft Production in England. Also her press baron uncles Northcliffe and Rothermere (the latter had briefly given Hitler a positive slant in the Daily Mail). All the time she stressed her Irish origins and loyalties. Later Peter told her that her performance had "tipped the scales". She says modestly: "In danger you are very alert so you don't make mistakes; you think, how will I get out of this?" Peter was released and sent to a punishment squad in the army, a fate far healthier than that which befell the others involved in the failed plot. In The Past is Myself, Bielenberg learns that they were hanged from meat hooks and filmed as they died. "They couldn't nail Peter down, that was it," she recalls. "All the others went through the People's Court, a ridiculous affair, but Peter never got to the court. His interrogater let him out and he was put in the camp."

During the war Bielenberg had little idea of the true extent of what was happening in other concentration camps, where Jews were being exterminated: "I'm always asked, what did I know. We were being bombed every night in Berlin; we didn't know if we would survive. I remember seeing less stars on coats. But you could only take in so much." She let a Jewish couple stay in her cellar for two nights, feeling physical pain from the shame that she could not risk sheltering them for longer. "The boys kept asking questions. It was just too difficult." Then there was the day when a friend saw a train with children's hands visible through the slats. "Even then we didn't know until after the war the real extent of what was happening, that they were being automatically gassed."

Bielenberg's post-war journalism left her with some vivid memories, including shocking lack of sympathy the Germans showed towards their soldiers. "The German boys came home having trudged through Russia, some wounded, no money. They were begging on the street. But the Germans blamed them for having lost the war, so they had little sympathy. I saw a boy rattling a tin mug in a doorway with one leg off. It was a terrible moment. I couldn't help him, I only had Allied money and I was in British uniform. But none of the Germans were stopping to help, or even talking to him." Christabel, like her mother, was named after Christabel Pankhurst, one of the early suffragettes. She has always had a keen interest in women's rights, mentioning that during the war she and so many other women were in effect, "single mothers". "Our husbands were in Russia or fighting. We had all the problems single mothers face now, what with bringing up sons on our own. I saw Peter maybe once or twice a year, for six years." Her perspective on feminism has changed now that she is "an old bird". "Women have a role to play and they are playing it now. But it would be a pity if they get too bossy and pleased with themselves."

Her interest in the women's movement and her desire for peace in Northern Ireland led her to visit Betty Williams and Mairead Corrigan (who later won the Nobel peace prize) in the 1970s. She describes the joy of those early peace rallies, when women from the North and South "crossed the divide together and convinced me that if peace and ceasing of bloodshed were to come about on this island it would be brought about by the women." She compares this ground-level work with the start of the post-war revival in Germany, also spearheaded by women.

She was interpreter for Betty Williams at Heidelberg University and recalls how Betty dealt with a heckler: "She took him firmly by the shoulders . . . and announced: `I'm going to speak and you're going to listen, d'y hear me, y' wee f-ker!' "

Bielenberg believes that in spite of the "wangling" of Northern politicians, "the people still want peace". She thinks Ireland should be united "in some way": "It would be so interesting. But there is that funny little enclave of Northern Protestants, most of whom come originally from Scotland."

She grew up with a strong family loyalty to Ireland: "My father's mother was very nationalist. She had to go to military dinners, because all her sons were in the British army, but she refused to toast the king and queen." Of her own sense of identity, Bielenberg shrugs: "I've been such a mixture all my life, these things interest you but you don't get too emotionally involved. By nature I'm Irish, that's how I feel." As for her three sons (all of whom live near her and have produced a populous brood of grandchildren and great-grandchildren): "They don't feel any one nationality, but connected to all."

She has positive feelings about the way the century has evolved in her long lifetime. She is pleased that the rigid class differences she grew up with are now falling away. She laughs as she recalls being presented at court: "We all wore white dresses and three feathers in our hair. We had to curtsey to Queen Mary and the king and two of us fell over. It wasn't easy, curtseying with a train. An American girl ahead of me got sick - we all had bouquets and there was an awful smell of gardenias - but the queen didn't even move. A guard stood over the sick." She also believes that there could never be another world war on the scale of the one she lived through: "Technology has reached such a stage that we couldn't. And the media has changed things. It has brought people closer together. In my day it was primitive and divisive: the French were called frogs, the Italians were the Eye-ties, and England ruled the waves. Nowadays you know that people are people. Whether they are French or German is not what makes the difference. You either like them or you don't."

In her own case: "I go out to like people. I went out to like the Germans, and that was hard; they are so reserved. It has been so much easier in Ireland. The Irish have something that is very attractive, they are intelligent and they have a gift for words."

She had what she describes as "a simple Christian upbringing that provided me with a sturdy conviction of the ultimate triumph of good over evil". The second in a family of two boys and two girls, she was taught to believe that what was right and what was wrong was a straightforward matter: "My mother was down-to-earth. I remember her refusing to give a donation to a man who was collecting for the foreign missions, saying: "I don't agree in trying to turn good Muslims into bad Christians'."

Bielenberg's experiences in Germany tested this early moral foundation: "In a regime such as Hitler's, there could be no standing on the sidelines, but there were also no rules to the game; each to his own conscience, that silver thread which must run through people's lives, ruling how far they should go and no further."

Fortunately her own conscience had been independent and self-willed from an early age: "My sister and I were at boarding school. She got expelled, and I was due to be head girl. I was told if I couldn't control my sister, I wouldn't be able to control the school. I loved my sister, and this treatment made me hate the school. I left and got a scholarship to Oxford on my own."

The Past is Myself has sold nearly a million copies and brought laurels: a Commander's Cross of the Order of Merit from Germany; a Golden Medal of Merit from the European Parliament; chosen by Time Life as a World War Two Classic; and a TV film version starring Elizabeth Hurley. She has also written a sequel, The Road Ahead, based on the years just after the war. All of this success has brought her a lot of pleasure, but "the most important thing is that people are still reading my books and writing me letters all the time".

As for now, she reluctantly agrees that although "people are bullying me about writing another book", perhaps it's alright to rest on her laurels, just a little.