The artefact — known as the Morpeth Roll — contains the signatures of more than 160,000 Irish people paying tribute to George Howard, or Lord Viscount Morpeth, when he left his post as chief secretary of Ireland in 1841.
It measures 420m long — three times the length of Croke Park — and is being published online after a four-year project to digitise the testimonial.
Researchers trying to uncover the stories behind the signatories, including aristocrats, merchants, politicians, and clergy, discovered the connection between Britain’s royal family and a man on the roll called Henry White, from Booterstown, south Dublin.
One of three brothers, Mr White’s great-grandson Luke Henry White married Lavinia Spencer, sister to Princess Diana’s grandfather Albert Spencer, in 1919.
Deirdre Watters of National University of Ireland Maynooth, which worked on the project, said the link was only discovered in recent days and research was ongoing.
“Henry’s father Luke White appears to have made his money in publishing companies,” she said.
“We think his is a bit of a rags-to-riches story. They would have been nouveau riche at the time.”
As they rose to prominence, the Whites bought Luttrellstown Castle, on the edge of Dublin’s Phoenix Park, where David and Victoria Beckham got married.
The Morpeth Roll was a rare tribute to an English administrator in Ireland.
It read: “We assure Your Lordship that the warmest Good Wishes of our Country will ever Accompany You, in Your Future Progress through Life.”
As well as being one of history’s longest records, the roll is seen as being of huge significance because it pre-dates the Great Famine. Most of the Irish census records were destroyed during the Civil War.
Genealogists say it is one of the few primary resources left detailing people living in Ireland during the 1840s and will prove invaluable for anyone researching their Irish ancestry.
Well-known figures who signed the roll include Irish political leader and emancipator Daniel O’Connell, entrepreneur and transport pioneer Charles Bianconi, and Irish nationalists Thomas Davis and Charles Gavan Duffy.
For years, the Morpeth Roll remained hidden away in a basement at Castle Howard in Yorkshire — the ancestral home of Lord Viscount Morpeth.
The artefact will be on view to the public for the first time in 170 years as it is taken on a 14-month tour of Ireland to coincide with its online release.