The unsolved theft of the Irish Crown Jewels particularly fascinatesconspiracy theorists. A new book on the saga links it to an extensive homosexual network and a plan by unionists to thwart Home Rule legislation, writes Robert O'Byrne.

On the morning of July 6th, 1907, four days before the arrival of King Edward VII in this country, it was discovered that the Irish Crown Jewels had been stolen from Dublin Castle.

This was a matter of some embarrassment to the Lord Lieutenant at the time, the Earl of Aberdeen, and, even more so, to Sir Arthur Vicars, who held the post of Ulster King of Arms and was responsible for the jewels' security. The items were kept in a safe in the castle's Bedford Tower, which also accommodated the offices of Sir Arthur and his staff.

What quickly became apparent after the initial discovery of the jewels' absence was that their theft had taken place some time earlier, possibly even weeks before, but no one had noticed. It seems the thieves were so keen to have their work discovered that they had returned more than once in order to leave behind successively more obvious clues that a break-in had occurred. The administrative incompetence in Dublin Castle made evident by the theft was compounded by subsequent official attempts to discover who was responsible for the crime.

It remains unsolved. The jewels were never recovered, despite the most extraordinary efforts during the past 90 years. In the immediate aftermath, for example, Sir Arthur Vicars consulted a psychic, who advised him to search graveyards in the vicinity of Clonsilla, Co Dublin.

More recently, two years ago on British television, presenter Carol Vorderman tried to work out what had happened to the jewels - with just as little success.

At least some of the fascination this theft continues to exert arises from the concept of the Irish Crown Jewels. In fact, the pieces were far less interesting, or valuable, than their name suggests. There were only two items of any importance, and even these were worth relatively little, certainly by comparison with their English equivalents.

The jewels arrived at Dublin Castle after the death of George IV, when they were returned to the state authorities by the king's last mistress, Lady Conyngham.

Originally, they had belonged to Queen Charlotte, wife of George III, who in 1783 had created the Most Illustrious Order of St Patrick, which would have 22 Irish knights. In 1830, when Lady Conyngham gave back the jewellery, it was decided the stones should be used to create insignia for the order: an eight-pointed star of Brazilian diamonds with an emerald shamrock and ruby cross at its centre; and a similar badge of rubies and diamonds as well as a gold harp and crown.

These were the jewels taken from Dublin Castle, along with a number of other pieces, including £1,500 worth of jewellery that Sir Arthur had inherited from his mother. It now seems almost certain that at least part of the intention was to embarrass the British government on the eve of a royal visit to Ireland.

Nevertheless, because the crime has not been solved, it has given rise to all sorts of notions ab- out who might have been responsible and why. The subject has proved especially popular with conspiracy theorists.

Four years ago, the Sunday Mirror blamed the theft on the Irish Republican Brotherhood, claiming that this organisation blackmailed two heralds within the castle to carry out the robbery and then murdered them. This newspaper article also announced that the jewels had been buried near Greystones, Co Wicklow.

Meanwhile, a new book on the disappearance of the Irish Crown Jewels comes up with an even more elaborate plot based on an extensive homosexual network and a plan by unionists to thwart the Home Rule legislation being proposed at the time.

The authors believe the jewels were recovered relatively soon after their disappearance and exist to this day. Perhaps so, but the more probable story is that the pieces were taken out of this country and broken up for the value of their constituent parts.

Much more interesting than any of the theories that have been put forward to explain the jewels' loss is the cast of people associated, either directly or obliquely, with the crime.

Lord Ronald Gower, for example, was a good friend of Sir Arthur and his circle; he had earlier been close to Oscar Wilde and, incidentally, was an avid correspondent with the young Sir Hugh Lane.

The son, brother and uncle of successive dukes of Sutherland, Gower was a talented sculptor - his statue of Shakespeare can be seen opposite the Swan Theatre in Stratford-upon-Avon - and among his lovers was the American, Morton Fullerton, who would later be the great passion of novelist Edith Wharton.

But unquestionably the most curious character in the Irish Crown Jewels saga was Frank Shackleton. The younger brother of polar explorer Ernest Shackleton, he was appointed to the largely honorary post of Dublin Herald by Sir Arthur, with whom he shared a house in Clonskeagh for a number of years.

Shackleton's financial affairs were always suspect and even at the time he was considered a prime suspect in the jewellery theft. A few years later, he was sentenced to 15 months' hard labour on a charge of fraud; eventually, he changed his surname to Mellor and settled in Chichester, where he ran an antiques business. Understandably, in all the recent publicity given to Ernest Shackleton and his Antarctic voyages, Frank Shackleton has not been much mentioned.

• Scandal and Betrayal: Shackleton and the Irish Crown Jewels by John Cafferky and Kevin Hannafin is published today by The Collins Press, priced €15