Jean O’Neill never attended university — or even school, being privately educated at home — but became a dedicated amateur plantswoman whose plant-recognition skills astounded even professional horticulturists and botanists. To some, her facility seemed almost instinctive, but it was the product of wide experience, a phenomenal memory and an eye for seeing crucial similarities between seemingly different plants.

She learnt the techniques of plant identification at an early age, growing up in a country house with extensive gardens that were abundantly stocked with interesting plants, and where she enjoyed the tutelage of a horticulturally minded father and his head gardener. She practised her craft with increasing passion, even while combining it with marriage to the Ulster Unionist politician Terence O’Neill, Northern Ireland’s fourth Prime Minister, 1963-69, of whom she was highly supportive.

Gracious, tactful and with a genuine interest in people, she forged enduring friendships with those she met through membership of bodies such as the National Trust’s Gardens Committee (whose chairman she became) and the Garden History Society (of which she was a vice-president). The same characteristics also served her well as a prime ministerial consort in the early days of the Troubles. “She was such a sweet lady, courteous and kind, yet behind this gentleness was a shrewd and active mind to the last,” one friend, the horticultural broadcaster Roy Lancaster, said.

For O’Neill, giving her gardening friends a tour of her one-acre plot in the grounds of her childhood home could easily take half a day. They would progress only a few feet at a time because it was a garden not only of rare plants but of memories, and wise friends would prompt her to tell the stories of where she had first seen the plants in the wild, who had given her the cuttings or how the seed had been saved. She, too, was more than generous with seed and cuttings, an inveterate plant-swapper.

An assiduous collector of plants from overseas, she made a special study of the English amateur botanist Peter Collinson, who, two centuries earlier, had had seeds, bulbs and cuttings sent to him from America. Peter Collinson and the 18th Century Natural History Exchange — a book that she co-wrote with an American writer, Elizabeth P. McLean — is due to be published this year by the American Philosophical Society.

Born Katharine Jean Whitaker into a family whose fortune had derived from the making of Marsala wine in Sicily, O’Neill grew up at Pylewell Park, near Lymington, Hampshire, where the mild climate and free-draining acid facilitated the growing of exotic plants — and thus were laid the foundations for her passion for international plant collecting. An unwilling debutante, she spent much time as a young woman sailing her boat — one of a class quaintly known as the Lymington Pram — around the Solent. She wittily christened this modest vessel the False Alarm, in deference to her uncle’s rather more prestigious J-Class boat, the Alarm.

During the Second World War she served as a Voluntary Aid Detachment (VAD) nurse, and in 1944 she married Terence O'Neill, then an Irish Guards officer. When the war ended the O’Neills settled near Ballymena, Co Antrim. He became a Unionist MP at Stormont, and she not only an MP’s wife and the mother of their two young children but the custodian of a five-acre garden who also managed to find time for good works. She was heavily involved in the National Trust’s restoration of its garden at Rowallane, Co Down, and advised on the restoration of two of the trust’s other Northern Irish gardens, at Mount Stewart and Castle Ward. She was also a fellow of the Linnean Society and a keen supporter of the International Dendrology Society, travelling on many of its overseas tours.

Her researches into the man who was in many ways her 18th-century role model, Peter Collinson, first saw the light of day in 1981 in one of a series of articles she wrote for Country Life. She quoted with approval Collinson’s appreciation of the help he received from an American plantsman, John Custis: “What much enhances the Obligation on my side is that being an Intire Stranger you should take so much pains to gratifie Mee.” This was a sentiment with which she could associate, having herself received much help from American plantsmen and women, some of it when accompanying her husband on prime ministerial trips to the US.

When Terence O’Neill retired from Stormont politics in 1970 (and was created a life peer as Baron O’Neill of the Maine), she moved with him to Lisle Court Cottage — in the grounds of her childhood home, Pylewell Park — where they lived together until his death 20 years later. At Lisle Court she created a one-acre garden of rare plants, many of which she germinated from seeds she had collected in the wild. Her fascination with Antipodean plants was always much in evidence after her son moved to Australia, but at Lisle Court the Australian Grevilleas and Lomatias still had to jostle for space with Latin American Puya chilensis or Beschorneria yuccoides because she was a tireless traveller in pursuit of plants until well into her eighties.

Her last big expedition, at the age of 88, was a two-week motorboat excursion down the Amazon, led by a friend, Professor Sir Ghillean Prance, former director of the Royal Botanic Gardens, Kew. On this trip she was surprised but not much daunted to find an Amazonian giant otter asleep on her bed one night. She also took in her stride a break-in by three masked men when she was alone at home a few years after her husband’s death. Tied up and threatened, she so impressed one of the three with her charm and composure under stress that he apologised, tried to make her comfortable, telephoned the police to ensure that they lost no time in releasing her, and — she thought — must even have been responsible for a cryptic card she received the following Christmas. A devout Christian, she had no difficulty in forgiving all three of her burglars.

Earlier this month a small group of O’Neill’s gardening friends set out to preserve the best of her work at Lisle Court by taking cuttings of some of her rarest plants to grow in other important gardens such as Abbotsbury, Dorset and Exbury, Hampshire. One of the group, Roy Lancaster, will also investigate whether a Grevillea from her garden is as yet unnamed. “If it is, I have it in mind to name it after her,” he said. There is already an Epimedium Jean O’Neill, named by another friend, Peter Chappell.

In 2000 O’Neill was awarded a Veitch Memorial Medal by the Royal Horticultural Society.

She is survived by her son and daughter.

Lady O’Neill of the Maine, plantswoman, was born on January 16, 1915. She died on July 15, 2008, aged 93