January is surely the harshest test for any garden. Yet the sun shines, albeit with a wintry muted light, on Helen Dillon's masterpiece at 45 Sandford Road in Dublin's Ranelagh. It is a study in elegance which testifies to the harmony of art and science within nature. On any day, in all weathers, there are at least seven gardens to view here, with hundreds of variations in the course of a year. Dillon, a relentless perfectionist who will be honoured in June by Britain's Royal Horticultural Society, patrols her garden with a mother's strict, loving eye.

Unlike many master gardeners, she is generous with her knowledge. Within moments of meeting her she has mentioned her two mentors: the late David Shackleton, once the master of another famous Dublin walled garden, that of Beech Park, Clonsilla; and Graham Stuart Thomas, author of the classic Perennial Garden Plants or the Modern Florilegium (1976), of whom she says: "He's a gardener as well a brilliant plantsman." A bearded light blue Iris he gave to her resides here.

For all the formality of her garden, with its magnificent centrepiece, a perfect lawn, it has not been beaten into submission. Instead, it displays a relaxed if disciplined personality, having been more indulged than tamed. Admittedly there are no traces of children or dogs rampaging through it either. Sir Reginald, the Dillons' dachshund, is built so close to the ground he tends to avoid wet grass and foliage. While Dillon carefully sweeps the slightest few particles of earth into the nearest flower bed with her hand, her eye ever searching for any possible imperfection, she sustains a conversation ranging from the manifold delights of hellebores, to the first sighting "in the 28 years I've known this garden, of a goldcrest" and her pride in having now survived a year without smoking. "And I was a 70-a-day woman; I was really disgusting."

Immaculate and already coming to life, the Dillon garden, which in fact never sleeps, should intimidate. Instead it beguiles, because an artfully deliberate attention to texture, shape and colour softens its potential austerity. An ingenious system of cleverly concealed narrow stone paths dissect the beds, allowing close inspection of the plants without trampling them.

The famously candid Dillon makes no secret of the fact that the plants are her first priority, and should they be threatened by a photographer's ladder or a carelessly placed foot or a hand straying too close for comfort, she will object. Her approach is direct, funny in a brisk "don't you know" way and colourful as viewers of RTE's now defunct gardening programme are aware. A television natural ("I love the buzz") she can pass on an immense amount of information without becoming pedantic. Dillon does not believe in talking down to concerned amateurs. Humour is central to her formula for informing without intimidating.

Often giving the impression she is absent-mindedly speaking to herself - "the thing to remember is that you don't need 472 different hostas" - as much as to anyone else, she prefers to deflect the seriousness of her dedication with humour. While not unsympathetic, Dillon zealously maintains no garden plant simply dies; it is usually helped on its way to eternity by its gardener, as often a caring one as a neglectful one. She does not absolve herself of routine garden murders either. "One of the most spectacular plants I ever killed," she confessed in April 1993, "was a peony, Paeonia obovata var. alba. Nowadays we seem to be confronted by two main varieties of gardening expert: the po-faced scientist who enjoys making it difficult for the less skilled, versus the overly laid-back media man who pretends it all happens on its own. Dillon never disguises the work involved: "I love the digging." Nor does she understate practicalities such as the importance of tending, enriching and indeed changing the soil, never mind nurturing the plants. A garden can live or die by the quality of the compost. Beyond the impressive Victorian-style, wrought-iron gates, is a small brick courtyard which leads to the garden. Two facing mounds testify to the importance placed by the Dillons on the use of compost. On the left is the raw heap, still rather refined by normal standards. Much of the kitchen waste, dead flowers and leaves which go on this pile are shredded in advance and added to the grass cuttings. In time, as the contents decompose, they are transferred to a facing heap where the process of gradual break down and rot continues, eventually producing beautiful, rich compost.

As with the lawn, the compost is the creation of Val Dillon, who is extremely knowledgable in his own right yet always defers to his wife on matters horticultural. Explaining his system for the manufacture and use of compost he says: "The two bunkers work this way, when one is full, the other is low or near-empty." It is impossible to resist running one's fingers through the Dillon compost, itself a thing of beauty. Is it any wonder anything planted here achieves such results?

Above all, she loves plants and is an obsessive collector whose trophies include a small striped winter plant, the stink pod, or Scoliopus bigelowii. A member of the Liliaceae or Trilliaceae, it is a native of California and Oregon and blooms in February. "You can say I've been gardening for 50 years," she says and shows no signs of having lost her curiosity and enjoyment. Aside from working on her garden, writing books and lecturing, Dillon also actively contributes to horticulture by visiting derelict gardens in search of old-fashioned and particularly garden forms of plants in need of rescue for posterity. In 1976 she began keeping a record of every plant planted at Sandford Road.

For all the beauty of gardening, it is as important an aspect of social history as period houses. Her first book, In an Irish Garden (1986), which Dillon co-edited with Sybil Connolly, featured an array of mainly private gardens owned by a generation of gardeners who were in many cases the inheritors of the best of Victorian plantsmen values. Most of them, such as the Americans Nancy Dunraven and Henry McIlhenny are now dead - as indeed is Connolly herself - and McIlhenny's Glenveagh Castle and Park in Co Donegal is owned by the State. It also features that most overlooked of national institutions, the Botanic Gardens. The book, in which the gardeners describe their individual gardens, is a valuable document recording the development of Irish gardening.

"So much has changed. Up to about 10 years ago," says Dillon, "gardening was very fuddy-duddy. It was for the middle-aged and, let's be honest, in Ireland it was a largely Protestant pursuit, as was natural history in general. And there was that class thing. That's all different now. Gardening is trendy. It's suddenly okay to do it."

While she has written so much about the subject - including The Flower Garden (1993) and Garden Artistry - Secrets of Planting and Designing a Small Garden (1995), and a gardening column she wrote for three-and-a-half years in the Sunday Tribune, using her own garden as her main source - she admits to being wary of writing about other people's gardens. "I don't like doing it. It's like writing about a person's soul." But she certainly believes that any serious gardener should make a point of visiting other gardens and she has heightened our awareness of Irish gardens. Which does she most admire?

"In the North, Mount Stewart." Situated on the edge of Strangford Lough on the western shore of the Ards Peninsula in Co Down, the 80-acre site is largely the creation of Edith, Lady Londonderry, who arrived there in 1919 and between then and her death in 1959 created a garden which is also a world. The broad, thickly planted borders flanking Dillon's lawn reflect her regard for the colour combinations of Mount Stewart.