'I think privacy is one of the most important things in the world," observes Lady Beit. For this reason she has never before given a newspaper interview. But 2002 marks half a century since Lady Beit and her late husband, Sir Alfred, came to live at Russborough, Co Wicklow, and so, at the age of 86, she has accepted a small breach of her privacy.

Before Sir Alfred Beit bought the house in autumn 1952, both he and his wife had seen only photographs of the place. At the time, they were living in South Africa and she remembers telling Sir Alfred before he set off for Ireland that she really didn't like houses with terraces in front of them and that "I did hope it wouldn't be too big. I must admit when I got here and saw Russborough, I was daunted by the size. But the house has got the most friendly atmosphere and is completely unhaunted".

At the time of their move to Ireland, the Beits had been married for 13 years. The couple first met in November 1938 and their wedding took place the following April. Sir Alfred would have been regarded as a very eligible bachelor, a handsome 36-year-old Tory MP who had inherited a large fortune from his father, Sir Otto Beit. Clementine Freeman-Mitford was a first cousin of the well-known Mitford sisters; she possesses the family's blue eyes and charm, and uses both to great effect. Her mother was a first cousin of Winston Churchill's wife, who was also called Clementine.

In 1937, two years before their marriage, Sir Alfred bought and restored a large house in Kensington Palace Gardens, London. Here he had installed a fireplace copied from the one in the dining room at Russborough, a house he admired greatly after seeing photographs of it in Country Life magazine. Kensington Palace Gardens accommodated part of the Beit art collect-ion originally assembled by Sir Alfred's uncle and father. Among this group of pictures was the portrait of Dona Antonia de Zarate by Goya. "That painting means a great deal to me for two reasons," says Lady Beit. "Alfred was standing beneath it when he proposed to me in the house, and then during the Dugdale raid we were tied up beneath it."

Because of the outbreak of war, the Beits spent very little time in their London home and when peace was declared, they decided to go to South Africa after Sir Alfred had lost his seat in the 1945 British general election. Over the next seven years, according to Lady Beit, "apartheid became appalling. We madly disagreed with the government policies and I became increasingly disenchanted, so since we weren't particularly settled there, I said to Alfred that I'd like to come back home."

And home could be Ireland because Lady Beit has Irish ancestry; her maternal grandmother Mabell, Countess of Airlie, was a daughter of the Earl of Arran and had grown up at Castle Gore in Co Mayo.

By this time, following his mother's death in 1947, Sir Alfred had inherited the entire Beit picture collection, and the couple were keen to find a house in which it could be shown to its best advantage. Fortunately, in 1952 Col Denis Daly who had lived in Russborough for the previous two decades, decided to put the house on the market. "So Alfred got on an airplane and went over to Ireland. I couldn't come with him because I was working for an organisation called the Institute of Race Relations. But he telephoned me quite a bit and bought Russborough."

A contemporary report in The Irish Times notes that the price paid for the house and surrounding 550 acres was "believed to be more than £47,000".

Lady Beit remains keen to pay tribute to the Dalys who had saved the house from potential destruction during a period when a lot of other fine properties in Ireland were either demolished or left to fall down. Still, the Beits had to undertake a considerable programme of restoration. "We did quite a lot of structural stuff. The roof needed attention, we installed better central heating and plumbing and - very important - there was ghastly Victorian plate glass in all the windows, so we put in proper fenestration for the whole house."

Built outside Blessington to the designs of Richard Castle for Dublin brewer Joseph Leeson (later first Earl of Milltown), Russborough has long been proclaimed one of the most beautiful 18th-century structures in Ireland. When nearing completion in 1748, it was already called "a noble new house forming into perfection" and behind its 700-foot façade - the longest in this country - are rooms filled with wonderful rococo stuccowork by the Francini brothers. Lady Beit's favourite room was always the library "because I'm a mad bibliophile and adore being surrounded by books".

Inside Russborough, the couple installed their superlative collection of furniture, objets d'art and paintings. In the drawing room, for example, they reinstated into their original frames four marine scenes that had been commissioned from Joseph Vernet for Russborough in 1749 but were sold to an American collector during the early part of the 20th century. Elsewhere in the house was hung masterly work by such artists as Vermeer, Frans Hals, Velasquez, Murillo, Metsu and Gainsborough. Such is the value of the pictures that since 1974 Russborough has been robbed on three occasions; Lady Beit resolutely does not speak of these incidents.

Instead, she prefers to remember the pleasure that living in the house provided for herself and her husband. Always hospitable, the couple were happy to share that pleasure with many other people. "We'd lots of friends here already. I suppose they were mostly Anglo-Irish, let's put it that way."

But the couple also became immersed in Irish cultural life. "The moment we got to Ireland, we heard about the Wexford opera festival which had just started," Lady Beit remembers. "Alfred, who adored opera, immediately went there, became a good friend of Dr Tom Walsh and was soon co-opted onto the board." Lady Beit still travels to the festival every year and believes that "interest in the arts in Ireland now is far greater than it used to be".

Having no children, she and her husband eventually grew concerned for Russborough's future and offered it to one of Sir Alfred's nephews. When the latter regretfully declined this responsibility, in 1976 the Beits opted to create an artistic foundation for the benefit of the Irish nation and to ensure the future of the house and surrounding land. "When we started the whole thing up, we endowed it with some funds. The government was only too pleased because we didn't ask for any money."

In 1978, the house was first opened to the public. Less than a decade later, part of the picture collection was donated to the National Gallery of Ireland, where last year a wing was named after the Beits. Lady Beit likes to point out that this is the second time the gallery has benefited from the generosity of Russborough's owners; following the death of the sixth and last Earl of Milltown, in 1902 his widow donated the house's contents to the same institution. "I do think the relationship through the ages between Russborough and the gallery is incredibly interesting," Lady Beit remarks.

This relationship has been advantageous for the National Gallery too. When it was first announced in 1986, the gallery's then-director Homan Potterton described the Beit's gift of 17 paintings as being "probably one of the most magnificent ever received by any museum anywhere". As a gesture of thanks from the state for their generosity, the Beits were given honorary Irish citizenship.

Sir Alfred served on the National Gallery's board for more than 25 years "as he was really very knowledgeable about pictures", says his widow. Following his death, Lady Beit also joined the board but has since resigned the position "because I don't think people should go on for too long".

After 55 years of marriage, her husband died in May 1994 at the age of 91. At his funeral in Christ Church Cathedral, the then Archbishop of Dublin, Dr Donald Caird, commented that the couple had "obviously rejoiced in one another's love and constant companionship". Lady Beit tells how, when seated next to a young man who expressed surprise at the length of the couple's union, she replied: "Yes, and I can thoroughly recommend it."

Marriage clearly suited her very well but she has not disappeared into widowhood during the eight years since Sir Alfred's death. Russborough remains her home and her abiding interest. "Oh goodness, yes," she concurs. "Otherwise I wouldn't have stayed on here since 1994. Despite its size, the house is surprisingly cosy; let's just say that I have always found it both liveable and lovable."

•Russborough is now open daily to the public 10.30 a.m. to 5.30 p.m. until the end of September.

Thomas Pakenham, author and old friend: "Clementine Beit is far too modest to claim to be one of Ireland's greatest benefactors. But it was Clementine as well as Alfred Beit who rescued Russborough, filled it with treasures, and gave us all this astoundingly generous present."

William Finlay, Chairman of the Alfred Beit Foundation: "Clementine Beit's affection for Russborough and all it contains is extraordinary. It is only matched by her concern for those who are part of Russborough and their families.

"Her courage in the bad times and her enjoyment of the good times is typical of her good sense. She is what is nowadays known as something special."

Prof Kevin B. Nowlan: "Considering that they'd no Irish connections, it's wonderful that Clementine and Alfred took Ireland to heart, restored Russborough and gave their pictures to the National Gallery. They're the kind of friends any nation should love to have and cherish."