ABOUT 45 years ago a young Celtic poet moved to Sussex, England, to recover from an unspecified illness. There in a small thatched house, surrounded by countryside rich in folklore and history, she began writing a story to pass the time. Mary Poppins, - for that was what the story turned out to be - was published, Fringing her immediate and long-lasting fame.

Today P. L. Travers - known as Pamela Lyndon only to her closest friends - still writers. (Her latest book, Two Pairs of Shoes, will be published here by Viking in the fall.) Although the years have added up, Travers claims the creative muse visits her more frequently than ever. "Ideas come to me more and more vividly and richly as I go," she said in a recent interview in New York. "If I have the energy, I have a great deal still to write." She though a moment, then pulled herself erect. "And if suddenly I'm run over by a bus, O. K. I'll do it in another life."

As imperious and elusive as the nanny she immortalized "I don't think I'm a bit like Mary Poppins. She's on a much higher level.") Travers is no more tolerant of questions about her age than she is of any other personal questions. "I'm as old as my tongue and a little older than my teeth," she snapped. "I might be one hundred. I might be sixteen." She prefers, she said, looking down a slightly hooked, aristocratic nose, to discuss ideas.

Ideas come to her out of what she described as the "Great Caldron" - a swirling, timeless mass made up of all the myths, folktales, ideas, and perceptions from the beginning of mankind. "I'm a kind of douser. With a twig I find myths. How, I don't know. The twig trembles and there it is."

Two Pairs of Shoes, which is based on two ancient Middle Eastern tales, came out of the Caldron, as did Friend Monkey, a book published in 1971 (Harcourt Brace Jovanovich) which Travers claims tells all there is worth knowing about her. and, of course, Mary Poppins came out of there also. Travers continues to be amazed at the universality of the Mary Poppins figure who flies through the sky, dropping into situations when needed. she says she still gets letters from readers in foreign countries who ask if Poppins was modeled after their folk hero who also descends from above.

Travers grews up in Australia, the daughter of Scottish and Irish parents. Celtic legends were for her as common as her daily porridge. Bits and pieces of fairy tales often cropped up in her writing as a child, but it was not until she went to Dublin as a young poet to seek her fortune that she encountered well-known poets who used myths and mysticism with maturity, grace, and sophistication.

Dublin, when Travers arrived, was in the last years of its literary renaissance. Ideas and literary talk flowed as freely as stout in the city's pubs, and for a young writer it was equally intoxicating. "I don't think that can ever be duplicated," Travers said of those years in Ireland. "It was a vividness given only once in a lifetime. It was a new Athens. There is no new Athens now. They [the great poets] were doing more than writing. They were building . . . buidling up the ancient myths they had discovered."

She becamed good friends with Yeats and "intimate" with George Russell, better known as A. E. and was by Russell's side when he died in 1935. These men encouraged her writing and even published her poetry. "They licked me into shape as a writer, much as a mother cat licks her kitten into the shape of cats." Mary Poppins evidently pleased A. E., who is said to have told his protegee: "Poppins' cool green sex has captivated me completely."

With Poppins, its four sequels and the "Disney version (which she resisted untill the filmmaker agreed to use live characters rather than animated ones) Travers became stereotyped as a children's writer. It has proved a life-long nemesis. "I don't know why Mary Poppins is thought of as a children's book. [Had she had an umbrella she would have thumped it.] "Indeed, I don't think there are such things. There are simply books and some of them children read. Beatrix Potter, for instance. She said quite categorically, 'I write to please myself.' She was honest - for doesn't everybody do just that?"

Travers wrote in a recent article: "I seem to remember that I was grateful for books that did not speak to my childishness - book that treated me with respect, that spread out the stroy just as it was . . . and left me to deal with the matter as well as I could if they moralized I was not offended. I let them do it because of the story and promptly forgot the moral. . . So remembering my experience as a reader, could I as a writer speak to a child by way of his childishness?"

Although flatters Travers to have children with their pure, uncorrupted instincts select her books as their own, she gives the impression, nevetheless, that it pleases her even more to have adults like what she writes.

Travers hopes one day to slip out of the literary spotlight to join company with her favourite author, Anon. "Who knows," she said. "Maybe one day no one will know who wrote Mary Poppins."