Ireland's population was in "serious decline" before the arrival of the Vikings, according to new research.
It had been assumed that the Irish population saw a steady rise across the centuries until the Famine in the 1840s.
But academics at Queen's University Belfast have produced an estimate of past population numbers, which show there had been a decline for almost 200 years before the Vikings settled in Ireland in the 10th century.
They did this using rigorous data science algorithms, to analyse a database of archaeological sites discovered during the "Celtic Tiger" years, when there was a boom in motorway building and other developments.
Dr Rowan McLaughlin said Ireland's population appears to have gone into an unexplained decline around 700 AD.
"Millions of people lived in Ireland during prehistory and the earliest Christian times," he said.
"Around the year 700, this population in Ireland mysteriously entered a decline, perhaps because of war, famine, plague or political unrest.
"However, there was no single cause or one-off event, as the decline was a gradual process.
"The Vikings settled in Ireland in the 10th century, during the phase of decline and despite being few in number, they were more successful than the 'natives' in expanding their population.
"Today, genetic evidence suggests many Irish people have some Viking blood.
"This large database has opened up a completely new perspective on the past that we simply could not obtain any other way."
Emma Hannah, lead author of the paper who is taking the work further with her PhD research, added: "Often in archaeology we are focused on interpreting the evidence from a single site, but analysing quantities of data in this way allows us to think about the long term.
"Now we know these broad trends, we can better understand the details of everyday life."
The research from Queen's University School of Natural and Built Environment has been published in the Journal of Archaeological Science.
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