The bloodthirsty soldier slaughtered thousands when he arrived here in 1566 as a prelude to the Plantation.

His party piece was to sever the heads of those he captured and place them in long rows, like a wall, leading to his tent.

This was a potent warning to any chieftain who didn’t bow to English rule.

Gilbert didn’t distinguish between combatants and civilians. He’d happily lop off the heads of woman and children too, so that nobody would be left to bury the dead.

The skulls rotted and moss grew on them. Thus began the export of “cannibal medicine” from Ireland.

“One obvious reason is that the English felt less scruples about mutilating and cannibalising their subject neighbours than they did their own countrymen,” said Dr Richard Sugg, an eminent lecturer at Durham University.

His eight-year research on the export of thousands of human skulls from Ireland has culminated in the book Mummies, Cannibals and Vampires, which will be published by Routledge on June 28.

His research will also be the subject of a special Channel 4 documentary narrated by Tony Robinson, of Time Team fame.

“Long before the British Empire shipped living slaves to the West Indies, the Irish victims of English occupation became mercantile commodities after their deaths,” Dr Sugg said.

Plenty of money was to be made from the skulls, so much so that the English introduced an import tax of one shilling for each one.

“As late as 1778, the commodity [skulls] was still liable for duty and was also listed amongst goods which were imported into England, before being exported elsewhere.” Dr Sugg said.

He said Francis Bacon (1561-1626), the father of scientific inquiry, probably started the trend in consuming fresher skulls with moss growing on them. Up to then, English nobility had mashed up Egyptian mummies for medicines.

“Bacon noted that one could obtain skull-moss from the ‘heaps of slain bodies’ lying unburied over in Ireland,” Dr Sugg said.

A very eminent scientist later took up Bacon’s hint.

One summer the “father of chemistry”, Robert Boyle (1627-1691), was badly afflicted by nosebleeds. During a violent bleed, Boyle decided to use “some true moss of a dead man’s skull”, which had been sent from Ireland.

“The usual method was to insert the moss, often powdered, directly into one’s nostrils. But Boyle found that he was able to completely halt the bleeding merely by holding the moss in his hand, thus confirming that the moss could work at a distance.” Dr Sugg said.

Boyle was born in Lismore, Co Waterford, the son of Richard Boyle, 1st Earl of Cork and the richest man in Ireland. He later moved around Europe to further his scientific studies.

“For around 200 years, the human skull was a popular medical ingredient: grated or powdered, and often distilled, it was used against epilepsy, convulsions, and various diseases of the head,” Dr Sugg said.

Originally, the recipe for distilled powder of the skull was closely associated with the physician Jonathan Goddard (1617–1675).

However, Charles II (1630-1685) bought the recipe from him for possibly as much as much as £6,000.

“Charles probably distilled the drops himself in his own private laboratory. It [skull medicine] was requested by the Prince de Conti for his epileptic son in Paris and used once again on the deathbed of Queen Mary I,” Dr Sugg said.

In 1694, Pierre Pomet, who was the doctor to Louis XIV of France, wrote how “English druggists, especially those of London, sell the heads or skulls of the dead, upon which there is a little greenish moss”.

“The English druggists generally bring these heads from Ireland; that country having been remarkable for them ever since the Irish massacre.”

Dr Sugg said: “For once, this was not a massacre of the Irish, but by them — the Ulster rising of October 1641 which claimed the lives of perhaps 10,000 Protestant settlers. From the time of Cromwell it may have been very difficult to know if you were distilling or swallowing the skull of an Irishman or of an English settler.”