New research has found the population of Ireland was in "serious decline" when the vikings arrived here.
Researchers from Queen's University Belfast say the decline had happened over almost 200 years, in contrast to previous suggestions that the population had been increasing.
The decline is said to have been a gradual process, but was likely influenced by factors such as war, famine, plague and political unrest.
It's believed the vikings were more successful than the natives in expanding their population after they settled in Ireland during the tenth century - with genetic evidence now suggesting many Irish people have some viking blood.
According to data analysed as part of the research, the population decline could have been much worse without the arrival of the vikings.
The new study's findings have been published in the Journal of Archaeological Science.
Emma Hannah, lead author of the paper, said the research involved analysing a large database of archaeological sites discovered as a result of the boom in development during the 'Celtic Tiger' period.
The data has allowed for the development of a long-term population model for Ireland.
Ms Hannah explained: “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."
Dr Rowan McLaughlin, research fellow at Queen's, added: "This large database has opened up a completely new perspective on the past that we simply could not obtain any other way."