Architecture is the art of self-knowledge; it comes from within. For the onlooker, however, it is wholly external. Its practitioners are subjected to harsher criticism than artists involved in any other aspect of the arts. This is due to its visual immediacy, as well as representing a merging of science and aesthetics, as does music. However, musicians are not charged with the destruction of the landscape.

"Architects are not particularly popular," observes Valerie Mulvin with characteristic directness. But what has really happened is that it has become too easy to hold architects responsible for the sins of short-sighted planners and greedy developers.

Central to many current misconceptions about architecture in Ireland is a newly discovered love-affair with the past which has caused new buildings to be seen as a blatant rejection of romance, beauty and heritage. At this stage of our evolution it is difficult to locate truly natural landscapes - even the Burren bears the mark of ancient farming. Architecture should be seen as a crucial part of the evolution of society rather than a destructive influence.

At a time when it has become fashionable for couples to admit they can not possibly live together, Valerie Mulvin and Niall McCullough are not only married to each other, they work together - heading the Dublin architectural practice, McCullough and Mulvin, which has designed the exciting new extension of Trinity College Library. Light, space and natural ventilation in the reading areas were a priority in the project, which also presented the challenge of balancing old and new, which has become the architectural story of the ever-extending Trinity campus. Originally the college was essentially laid out as a series of courtyards - a fact to which these architects are most sensitive.

Their firm's design won the open competition from eight other contenders. McCullough and Mulvin are not interested in period pastiche; their designs are simple, strong and bold, and they agree that architecture should be "of its time". Restraint is also crucial as, McCullough says, "it is always important to know when to stop".

Of their library building he says: "It produces a corner-stone, it holds the geometry." He pauses for a moment before observing, "One of the most beautiful things about working in architecture is the mathematics of it." Their shared awareness of Ireland's rich architectural heritage is well documented in their co-authored study A Lost Tradition (1987, 1989), in which they explore the Irish architectural tradition from pre-history, through the Celtic and medieval period to the complex Renaissance era and on through to the emerging modern picture.

"We approached it as architects, not as archaeologists, nor as art historians," says McCullough. As ever, churches dominate as the most important public buildings in Ireland. The book is written with the same precision which dictates their conversation. For all their practicality - practical enough to cite energy and stamina as essential as inspiration - they are also imaginative visionaries. A Lost Tradition was inspired by their long-term romance with the Irish landscape, "built, unbuilt and ruined".

It is a superb book, combining photographs, line drawings and plans with an excellent text, which is as passionate as it is objective and informed, drawing attention to serious issues such as the yet-unexplored history of the classical vernacular style in Irish rural public housing, without resorting to polemic. The practical research took a year, "travelling on back-roads to 18th-century farmhouses, ruined churches and empty mills". In many ways, a pilgrimage of love, it is also an important act of reclamation. Notions of personal immortality do not appear to interest them, and McCullough refers to a recent Guardian interview featuring the British architect Cedric Price, who, at 64 is highly influential, yet has relatively few buildings still standing. Price is far more interested in the social and practical impact of his buildings than in creating personal monuments. In that article, Price remarked of the essential timing which is vital to architecture, and also stressed the greater flexibility architecture once enjoyed as a creative process.

"Only in this century have we failed to realise this," he said. "In the 17th century, the machine for living in was used and re-used, rebuilt and re-styled constantly. Things were left to ruin, entrances were changed. It hasn't always been the case that the client is satisfied with what he's been given, and will always want that."

McCullough and Mulvin were each drawn to the idea of becoming architects while still at school. Neither came from architectural backgrounds, although for McCullough the interest was inherited from his engineer father, while there has always been that love of maths.

Now 41, McCullough, an expansive character with a generous waistline and definitive views, at times still looks like a clever south Dublin schoolboy taking immense delight in information and ideas. Once he begins describing Dessau, the central German city which was an important design centre in the 1920s, and which has an extensive range of exciting buildings dating from that time, his initially formal manner disappears completely. His face, however, registers complete bewilderment when asked was he ever involved in sport either at school or college.

Niall McCullough is the eldest of four children. Valerie Mulvin, the eldest of six, spent her childhood partly in Dublin, partly in London. Her father qualified as a barrister but worked in insurance. "He would have liked to have been an engineer," says Mulvin. Her partner agrees and they refer to his love of model railways. McCullough and Mulvin met at University College Dublin.

For their final year projects, McCullough designed an agricultural research centre at Portarlington - he loves the flat midlands landscape, and the bog's juxtaposing of water and earth - and Mulvin designed a newspaper office. On graduating they explored Europe, investigating the glories of classical architecture which continue to inform their buildings. They also completed further study in Rome, where they worked for a time before returning to Dublin to establish their practice. Temple Bar Studio and Gallery is one of their most successful buildings, and when asked to define the creative pleasure of architecture, Mulvin says, "there is nothing as exciting as working with a three dimensional space".

How does an architect's vision develop? "In the first 10 years you do everything, then it's less and less." Is there an abiding essential criterion? "A building, any building, must be fit for the purpose for which it is intended," replies Mulvin.

Impressive public building and magnificent one-off homes for the wealthy, however, represent the more attractive face of architecture. For many of us, it is an elitist pursuit: rich people engage architects. Most observers nowadays are probably more concerned with the poor - in some cases, apparently non-existent - design in standard housing, particularly now, as suburban housing estates are being built in rural areas as well.

Why are the same unimaginative, uniform designs appearing throughout the country? "People buy them because they are there," she says. In response to a comment made about the current, near obsessional proliferation of hotels being erected in Dublin, McCullough shrugs, describing it as a "tax thing", while Mulvin wonders who is coming to fill them, and why it is still so difficult to get a hotel room in this city?

Rain is threatening, but the trees lining Dublin's leafy Leeson Park ensure that it remains one of the city's most beautiful and serene roads. Valerie Mulvin stands on the front steps of the fine mid-Victorian house which serves as their home as well as the offices of their practice, which now includes 10 architects: "We work downstairs."

Thin and slightly other-worldly, she is dressed in black, and her calm, intense face is reminiscent of a pioneering Irish architect from an earlier generation, Eileen Gray. Mulvin is 43, and the mother of a 23-year-old son. She uses the word "context" a lot and creates the impression of being able to make most things possible.

The classic adjoining reception rooms are graced with typical Victorian proportions and that vaguely uninhabited quality that architects tend to bequeath to rooms. As expected, everything is white: the walls, the doors - even the floor, which is stained white. The walls are bare except for two Fergus Martin pictures, recent purchases. "It's not finished," she says of the house in general. The homes of architects seldom are.

Sitting at the rear-garden end of the rooms and looking towards the front of the house offers a fine view of a large plane tree, which is visible through both windows, and appears to be neatly bisected by the wall between. The light from the garden is watery, soft. The scene seems too peaceful to spoil with conversation, but rather than make this more of an architectural consultation than an interview I break the ice . . . So, here we are in the centenary year of Frank Lloyd Wright. They both laugh and acknowledge his achievement in bringing something new to architecture.

One of the most appealing features of this cool, austere house is its impression of height. Having walked up the front steps, the reception rooms provide a viewing platform on to the world waiting outside. They have lived here for five years, but are far from giving the impression of being settled. When Mulvin remarks, as if on cue, that houses should move in five-year cycles, it does not surprise. Tea arrives in a small, defiant, pale green tin teapot, which looks as if it has quite a history of its own.

A model of their Trinity library waits patiently in the corner. McCullough places it on the tea table and begins to describe the space. They explain why the south facing wall is solid - in order to avoid over-heating. Their conversation is a dialogue of shared vision.

It becomes easier to imagine the full-scale building, complete with students inside. Once the model is brought in to the discussion, both architects begin speaking with their hands and their explanations become more vivid. It is also important to note the value they place on the mechanics as they watch a building emerge from plans on paper and begin to take shape.

"A large building site is an exciting place. We are constantly in touch with people who are good at what they do - plasterers, painters - it sustains your understanding of the ordinary."

Her strength is the abstract. Mulvin makes ideas acquire life. For all her intense exploration of the physical world, she seems to live in her imagination. They are deliberate, opinionated readers - particularly of fiction. She thinks de Lillo's Underworld is a masterpiece; he couldn't "get into it", but is prepared to give it a final shot. Their observations are highly literary.

It seems appropriate that McCullough should have quoted from Calvino's Invisible Cities in the introduction to Palimpsest - Change in the Irish Building Tradition (1994). "Traditional art history, concerned to draw a clear path, tends to emphasise the `perfect' work of art or architecture as the summation of all the effort towards which a generation has striven. Reality is more imperfect . . . each generation steps away from its built artefacts as in a hall of mirrors - early familiarity, contempt and dismissal, then the building as representative of an age, then curiosity where information is lost and later re-invention to suit a creative process of mythification."

For him "the history of architecture contains many juxtapositions which are not a true palimpsest of layers added over time, but echoes of survival and evolution. At the simplest level, one comes across artefacts from widely differing periods which appear similar in character - but merely represent the endurance of form by reason and structural logic or force of ritual." The evolution of the castle to the large country house is but one of Irish architecture's many intriguing tales.

His observations on the Irish house are particularly telling. "At the very largest scale, important country houses display two complementary phenomena - the absorption and disguise of earlier fabric in a classical or Palladian framework and subsequently, the reverse cannibalisation, the addition of Gothic windows, towers and castellations to classical house in an attempt to render them as Romantic objects in landscape. Occasionally, as at Tullynally Castle in Co Westmeath, all stages occur in succession - a solid planter house became a Georgian mansion and then an extended Gothic castle." Admittedly not the most romantic of houses. McCullough draws attention to Tintern Abbey in Co Wexford, a monastic building which was adapted for domestic use, as a remarkable monument to changing architectural tastes, "each evolution carried out in the framework of the monastery church".

Time and again, the listener is reassuringly aware that these architects who enthuse freely about the early 20th-century architecture of Chicago, and the shrewdly planned state architecture in Germany and Spain, are equally impressed by early Irish monastic building. There is a wonderful description of the tiny, dry-stone chapel known as Gallarus Oratory on the Dingle Peninsula in The Lost Tradition.

McCullough notes that the building's unique form is derived from the corbelled clochan: "an innately circular discipline pushed to its limits in the creation of a rectangular building suitable for Christian worship. It is all roof, rising from ground to apex like the frame of an unturned boat, dry-stone walls laid in ever-decreasing layers to a single line along the ridge. In a sense, it describes the beginning of experiments in stone roofing; it is really a link between the prehistoric and the Christian worlds, a technology as old as the Passage Graves reaching out to encapsulate new ideas of ritual space."

Theatre, cinema and the visual arts are also important to the couple. Mulvin, whose heroes range from the French historian Ferdnand Braudel to the Australian art critic and cultural commentator Robert Hughes, will illustrate a point in language which borders on the philisophical. McCullough is different, his examples are invariably textual. Few people could manage to put as much sheer information into their everyday conversation as he does without appearing - if not pedantic - at least professorial.

Neither of them is prepared to air any specific architectural pet hates. McCullough believes Ireland is currently living out planning mistakes made 20 years ago. "It takes time to sort things out," he says. Just as Valerie Mulvin points out, "It is too easy to dismiss everything that was built or done in the 1960s as wrong. Mistakes were made. But good things were done as well."