Ireland’s best-known name is Guinness, a global brand whose brewing origins have defined the way the country sees itself. But research conducted by a member of the Guinness family has discovered that they are not descended from the Gaelic aristocracy, as has always been maintained, but a near-slave class of Irish peasants.

Patrick Guinness, a financier and father of the former fashion model Jasmine, asked the genetics department at Trinity College Dublin to investigate the origins of his family.

The results, detailed in a book he has written about his famous ancestor Arthur, the founder of the Guinness Brewery in Dublin, rewrite the story of Ireland’s most famous family.

The biggest myth destroyed by Arthur’s Round: The Life and Times of Brewing Legend Arthur Guinness, published by Peter Owen, is that he and his father Richard were descended from the Magennis chieftains of Iveagh, Co Down.

In Irish the name was Mac Aonghusa, meaning son of Angus. Two years after opening his Dublin brewery in 1759 Arthur had a silver cup given as a wedding present engraved with the armorial bearings of the Magennises of Iveagh. When Edward Cecil Guinness was elevated to the peerage in 1890 it was as Baron Iveagh of Iveagh.

However, Patrick Guinness asked the genetics department of Trinity College Dublin to include the Magennis clan in a study that looked at the Y-chromosomes of more than 300 men with Gaelic East Ulster-origin surnames. Y-chromosomes, like surnames, are passed from father to son.

“It turned out that we weren’t descended from the Magennis chiefs at all,” Patrick Guinness told The Times. “Not only were we peasants rather than chiefs, but we were also more closely related to the next-door clan, the McCartans.

“The two clans lived side by side in Co Down. I then looked at the area from where they came and discovered a hamlet whose name is Gion Ais in Irish, meaning a wedge-shaped ridge.” The name was anglicised to Guinness or Ginnies.

He believes that the family name is, therefore, not a patronymic but a toponymic. “We took the name from a place and not a family.” Ginnies lies in hilly country six miles southwest of the town of Ballynahinch.

The discovery takes a hammer to Arthur Guinness’s reinvention of himself in 18th-century Dublin society as a high-born Gael who was reviving the fortunes of his aristocratic family after it suffered under Oliver Cromwell. The Earl of Iveagh, Bryan Magennis, fled abroad after the defeat of James II at the Battle of the Boyne in 1690, about the time that Arthur’s father, Richard, was born, and the clan’s lands confiscated.

“They [the Guinnesses] would have seen it as a link to a story of dispossession,” said Patrick Guinness.

“But in fact the Magennises worked on and off for the English administration in Ireland until that fell apart after the Boyne.

“The idea was that we were actually very grand, then we were dispossessed, then the Guinnesses got back on their feet in business and brewing.”

Patrick Guinness observed that, like many poor people moved on by Cromwell, his family gravitated towards Dublin rather than heading west with the richer families.

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