Eileen Gray is the mystery figure of 20th-century modern architecture. How did the Irish-Scots aristocrat from Enniscorthy, County Wexford, become an originator of Parisian art deco? Even stranger, how, without any training as an architect, did she reinvent herself as one of the superstars of quite a different style, the architect of modernist buildings of a reticence so powerful they take the breath away? The answer must lie in Gray's exceptional contrariness as a woman and as an artist. "To create," she once said, "one must first question everything."  There is a sense of metamorphosis in Gray's work. Her rooms are designed to be flexible, impermanent, responsive to the demands of the moment. Screens can be shifted to redefine the spaces. Little drawers pivot outwards to reveal their inner contents. A storage cupboard drops down to provide a writing surface. A mirror is suspended on a pole that swivels. All is movement, realignment, possibility.  The same is true of Gray's life. She was a springer of surprises. To start with, Gray, who took the lead in redefining architecture as a plausible profession for her sex, derives her surname not from her father but her mother, who in 1895 inherited the Scottish title Baroness Gray in her own right. Her father changed his name to Smith-Gray by royal licence and the four children were from then on known as Gray.  The unlikeliness continues with her choice of lacquer-making, an obscure oriental art that is technically hard to master, at a time when her contemporary craftswomen were concentrating on more conventional pursuits: bookbinding, weaving, embroidery, jewellery making. Nor was Gray ever involved in the arts and crafts guilds or supportive female groupings of the early 1900s. She always worked alone, and this fastidious apartness was the basis of her originality.  She studied at the Slade School of Design, but did not stay in England long. She had a wonderful ability to pack up and leave with no sentimental waverings. Gray had first gone to Paris in 1901, with two friends from the Slade, to take classes in drawing and returned to Paris permanently five years later, moving into the apartment in rue Bonaparte that she kept all her life. She was soon more Parisian than the Parisians, developing a style of decoration that came to be seen as quintessentially French.  Eileen Gray's work in Paris before the first world war is rarefied, exotic. She developed her skills in lacquer through meeting and learning from a young Japanese craftsman, Seizo Sugawara, who came from Jahoji, a village in northern Japan famous for its lacquer work. Gray filled her notebooks with colour recipes and methods of applying and layering the lacquer, seeing how she might draw on these ancient techniques to create a modern art form. Her screens and panels are abstract, with complex, swirling forms. The most marvellous of these, Le Destin (1914), is a four-fold screen in deep red lacquer on which a mysterious drama is enacted between two blue-grey naked youths and an old man wrapped in a misty silver shroud. Le Destin was acquired by the couturier Jacques Doucet, bibliophile and modern art collector, an early admirer of Gray's work.  After the war her interiors still tended to the sumptuous, though never to the vulgar. Gray's art deco was never lush or louche. She had a deeply moral disapproval of the overdone, covering over the "disgraceful mouldings" in the apartment in the Rue de Lota she was commissioned to design for Madame Mathieu Lévy, who, as Suzanne Talbot, was one of the most fashionable modistes in Paris at the time. Gray had been excited by Bakst's designs for Diaghilev's Ballets Russes, and the salon in the Lévy apartment is her own Schéhérazade, a finely judged ensemble of lacquered panels, matt gold cushions, a red lacquer serpent chair and one of her most beautiful designs, the Pirogue boat-bed, burnished like a barque for Cleopatra in lacquer and silver leaf. (The Pirogue was the show-stopper in the V&A museum's art deco exhibition of 2003.)  Very little is known of Gray's emotional life. Characteristically, she burnt all letters and personal mementoes in old age. But it seems that from her early years in France, Gray was on the fringes of the raffish intellectual Paris-Lesbos scene. She had a long, close friendship with Gaby Bloch, companion-manager of the American Marie Louise Fuller, alias Loïe, of twirling dance celebrity. She was later to be seen swanning around with Damia, the green-eyed, gravel-voiced singer whose popular "chansons dramatiques et tristes" had made her the Edith Piaf of her day. Gray escorted Damia to restaurants and nightclubs, elegant in her Poiret evening coats and Lanvin hats. She knew Nathalie Barney, Romaine Brooks and Gertrude Stein. But she never quite joined up with the Café Flore society of Americans in Paris. They were probably too raucous. The American photographer Thérèse Bonney recalled Gray's extreme inwardness: "Of all the people I knew in the world, she gave the feeling of complete consecration."  Gray's rebirth as total modernist began in the mid-1920s. By now she had opened her own shop in Paris, in the fashionable Rue du Faubourg St Honoré. The shop was called Galérie Jean Désert, the name of an invented male proprietor and reference to Gray's then-recent desert travels in north Africa with the weaver Evelyn Wyld. A female furniture designer in Paris was a rarity, but a cognoscenti customer base of writers, politicians, artists and architects was gradually created. Jean Désert sold the abstract geometric rugs designed by Gray and woven in Wyld's Rue Visconti workshops.  Her evolution was now recognised by other European modernists. Gray had her first major public showing in the Salon des Artistes Décorateurs in 1923. Although the sombrely sophisticated Bedroom-Boudoir de Monte Carlo was described by one critic as suitable for Dr Caligari's daughter, Gray was heartened to receive an enthusiastic postcard from the Dutch modernist architect JP Oud, a founder of De Stijl. Her subsequent display at the Salon d'Automne was praised by her fellow exhibitors Le Corbusier and Robert Mallet-Stevens. In 1924 the Dutch avant-garde magazine Wendingen referred to her as being "at the centre of the modern movement". She was seen as a designer with new visions for the future, demonstrating that "the formidable influence of technology has transformed our sensibilities".  Gray was the model of the self-made architect: "I started really by myself, sort of making plans of buildings." Her architecture evolved without the training or the tradition of the large office. She was never the woman in the shadow of the maestro, as Charlotte Perriand was with Le Corbusier or Lilly Reich with Mies van der Rohe. Her first house, named cryptically E-1027, was designed in collaboration with and indeed for her sometime lover, Jean Badovici, the Romanian architect and writer. But she did not allow him to get the upper hand. The plan was credited to "Eileen Gray and Jean Badovici". When the design was shown in a later exhibition as the work of Badovici with furnishings by Gray, on the assumption that women were by nature merely choosers of the curtains, she was understandably enraged.  E-1027 was built on the cliffs at Roquebrune, a clear white L-shaped house of exquisite simplicity. The south of France was then a favoured place for modernists, with its wild coast scenery and connotations of escape. Gray had loved the Riviera since she first discovered St Tropez, then quite undeveloped, on her early travels before the first world war. E-1027 developed as a "living organism", like the flora and fauna burgeoning around it. The furnishings were integral to the architecture, in Gray's apparently casual seaside "camping style". Most of the furniture for which Gray is now so famous - the Bibendum chair, the Transat chair - was designed for the new house. The E-1027 circular glass table, adjustable in height by means of a sliding pole, was evolved for Eileen's sister, who liked breakfasting in bed.  Though E-1027 was startlingly modern in its day, Gray was never a doctrinaire modernist. She stood apart from the dogmatic male dominated Congrès Internationaux d'Architecture Moderne (CIAM). What she achieved was different, and it was her own. Colin St John Wilson, architect of the British library and a Gray aficionado, has defined her "unique gift for turning the practical into the poetic", her intense "perceptions of habitability". Gray took the forms of modernism and invested them with a perfectionist tendresse. In the hall of E-1027, "Entrez Lentement" was spelled out with a kind of gentle jokiness. When she used industrial materials it was with the lightest touch. This makes all the crasser Le Corbusier's later desecration of E-1027 by painting nine large, lewdly coloured frescoes on the pristine walls, an act Gray called violation. Le Corbusier reputedly added insult to injury by painting in the nude.  Between 1932 and 1934 Gray built her own house, Tempe à Pailla, nearby at Castellar. She and Jean Badovici had by now parted. Gray continued planning buildings but little materialised.  It was only in the 1970s that Gray was rediscovered. Retrospectives in London and New York revealed her as a designer of intrepid intelligence. She attracted younger fans, among them Bruce Chatwin, who tracked her down in Paris. He remarked on two gouache maps of Patagonia she had painted on the walls of her apartment. Gray, by then old, blind and fragile, said: "Go there for me." When Chatwin returned, he wrote In Patagonia. Gray died in 1976 at the age of 98.  Can we really consider her as one of the giants of modernism on the strength of just two buildings? Gray herself, who regarded her late fame with a charming cynicism, would have said: "Mais c'est absurde." Yet her vision of the future - "the future projects light, the past only shadows" - was compelling. She humanised the architectural world. We need to see her as the precursor of such solo women architects as Eva Jiricna and Zaha Hadid. View Gray in broader terms as the builder of the home, literally the homemaker, and you have a potent image for the dawning feminism of the mid-20th century.  · An exhibition of Eileen Gray's work is at the Design Museum, London SE1, from September 17. Details: 0870 833 9955.
As visionary as she was contrary, Eileen Gray ranks among the giants of modernism, thanks to just two buildings.