Ireland's earliest Neolithic society had an elite ruling social class similar to Inca god-kings and Egyptian Pharaohs and were allowed to interbreed.
A team of archaeologists and geneticists, led by Trinity College Dublin, have shed new light on the earliest periods of Ireland's human history.
The findings were based on genetic analysis of the remains of an adult male found buried deep in the 5,000-year-old passage tomb at Newgrange, Co Meath.
Older than the pyramids, Newgrange is world-famous for its annual solar alignment, when the winter solstice sun- rise illuminates the inner chamber.
Until now, little was known about who was buried in the 200,000-tonne monument.
However, research published in the leading international journal Nature shows the genome of the man points to his parents being first-degree relatives - full siblings or a parent and offspring.
Universally, incest has been a taboo practice throughout history, but was socially acceptable among certain elite ruling classes - typically royal dynastic families.
The view was that this would strengthen their hierarchical position and legitimise their power.
"I'd never seen anything like it," said Trinity's Dr Lara Cassidy, the lead author of the paper.
"I was at my desk in the lab on my own when we got the first result. I had to reconfirm it a few times before I told anyone.
"I wanted to make sure I hadn't broken something. It's extremely rare, this type of mating. It tells us a lot more about the society into which this man was born.
"When you see this behaviour, it is almost exclusively elites.
"And even then just a royal family who take it to an extreme.
"We all inherit two copies of the genome, one from our mother and one from our father. This man was extremely inbred."
The study used genetic sequencing to reveal the Newgrange man was related to others buried in passage tombs more than 100km away in Carrowmore and Carrowkeel in Co Sligo, further pointing to a powerful social elite at the top of Irish Neolithic society.
"It seems what we have here is a powerful extended kin group who had access to elite burial sites in many regions of the island for at least half-a-millennium," said Dr Cassidy.
The Newgrange man's burial was heavily ritualised, and his tomb was placed in line with the winter solstice solar alignment.
The researchers sequenced 44 whole genomes from Irish Neolithic people, alongside relevant ancient genomes.
These were merged with an ancient dataset to allow for more detailed analysis, which found that overall there was no increase in in-breeding during the Neolithic period in Ireland.
This suggests non-elite communities maintained sufficient size and communication to avoid mating with their relatives.
The genome survey stretched over two millennia.