One of Lewis' best-known works is the The Lion, The Witch And The Wardrobe, which was made into a film in 2005, and his autobiographical Surprised By Joy was adapted for the highly successful play and movie titled Shadowlands.

Despite his international eminence as a writer and scholar, CS Lewis is still not a household name in his native land, in a way that almost everyone knows about other famous Ulster figures such as Seamus Heaney, Sir James Galway and George Best, and earlier names like Harry Ferguson and John Dunlop.

"Many people in Belfast still have not heard about CS Lewis," says Sandy Smith, who has been leading a series of weekend tours around the author's native city. "However, the relatively recent movie has led to a renewed interest in Lewis, and I am hoping that this series of tours will add to that."

They have been undertaken in co-operation with Belfast City Council, the Belfast Visitors' and Convention Bureau and the Northern Ireland Tourist Board, who wanted an added attraction for the passengers of four large cruise ships that have been coming into Belfast at weekends. Last month the Grand Princess docked in the city and a number of passengers joined Sandy's tour.

"There were Italians, as well as Americans who had read several of CS Lewis' books, but none of them had any idea that he came from Belfast. This shows that while Lewis is world-famous, a great deal of work still needs to be done to raise the profile of the city," he argues. The tours, however, are not just for visitors, and local people are welcome as well.

Sandy is a senior civil servant who has developed a passionate interest in CS Lewis, and he combines this with an almost encyclopaedic knowledge of the man and his writings. "I discovered that Lewis was brought up the same area of Belfast where I live, and that sparked off my interest," he reveals.

"Even though I have learned a great deal about him over the years, I am always learning more, and there are many allusions in his writings to the institutions of the Belfast of his boyhood. For example his references to 'the lion' probably derive from his connection to St Mark's, Dundela, which is known locally as 'The Lion On The Hill'. He calls his lion Aslan, and I have only recently found out that aslan is the Turkish word for a lion."

CS Lewis was baptised in St Mark's in 1899, and he and his older brother Warren installed a stained-glass window in the church in 1933, in memory of their parents. His writings also refer to the Belfast shipyard and to Campbell College, where he was a pupil until a respiratory illness forced him to leave Belfast and to seek a kinder climate in the health-resort of Malvern in Worcestershire. CS Lewis' father was a solicitor and his mother was the daughter of a Church of Ireland cleric. He was christened Clive Staples Lewis by his grandfather, who was then the rector of St Mark's, but at the age of four he announced that he wanted to be called Jack after a train-driver he knew.

For the rest of his life he was known to his close circle of family and friends as Jack Lewis.

Sandy says: "He was an Ulsterman through and through, and his love of Co Down in particular comes through in his writings. He started to write a novel about his early life in Ulster, but that was unfinished when he died. He also wrote a science fiction trilogy, but not many people are aware of that - including those who are knowledgeable about his other writings."

Lewis was also an eminent scholar, and he was a friend of JRR Tolkien, the author of The Lord Of The Rings. Both were members of the English faculty at Oxford University where Lewis held a Fellow of Magdalen College from 1925 until his departure for Cambridge in 1954, where he was Professor of Medieval and Renaissance Literature until his death in 1963. One of his scholarly contributions was to write Volume III of the Oxford Dictionary of English Literature, which he referred to privately as "O HELL".

Outside academia, he was well known for his children's fiction and his considerable range of Christian writings. Some people even regard him as a precursor of the fabulously successful JK Rowling, whose Harry Potter stories have an underlying distinction between good and evil, even though others would claim that the secularism in the world of Potter is predominant.

Sandy says: "Lewis writes children's myths to try to convey absolute truths, and he described some of his work in virtual one-liners. The lion, for example, was the Lion of Judah, which represented Christ, the witch was the embodiment of evil, and the wardrobe was the entrance to a whole new world through the imagination. His writing attracts those who may be unaware initially of its deeper meaning, but it has essentially a spiritual message."

Lewis' other works are aimed at an audience seeking a meaning to life, and part of the attraction is that his books are extremely well written, and they do not shirk from the big problems of human existence - including of pain and bereavement, in a world created by a 'loving' God.

"Lewis also asks, 'Are miracles possible?'," says Sandy, " and his answer is 'Most certainly, yes! There are things happening which may not seem plausible, but they do happen.' Lewis also asks, 'Does prayer work?' Again the answer is 'Yes' but not in a way that people might expect. Many turn to prayer in order to try to change their circumstances, but Lewis stresses that the essence of prayer lies in changing the attitudes of those who are praying."

Some of the deepest meanings in CS Lewis' work are discovered progressively by those who embark on the journey of finding out for themselves. Sandy explains: "This is why I am so keen that people get to know about Lewis in a wider context. Over the past few years I have led up to 20 tours in Belfast.

"Some of those who take part have little or no idea of who he was. I remember one tour when a visitor told me that he had literally come on board our bus because there was nothing else to do in Belfast on a wet Sunday morning.

"When I had finished, however, he asked me the way to the nearest bookstore, because he wanted to buy some of CS Lewis' publications.

"That, to me, was a real success. I am delighted in helping people to 'find' CS Lewis and to set them on their own journeys of discovery about the life and work of this remarkable man."

The last of Sandy Smith's CS Lewis Tours is taking place on Sunday, starting outside the Linenhall Library in Belfast at noon and lasting for just over 2 hours. They cost £7 and tickets can be booked through the Welcome Centre in the city centre, tel: 028 9024 6609