Ireland's saints and scholars were descended from farmers and bronze metalworkers from the Middle East and modern-day Ukraine, scientists have found.
Researchers have sequenced ancient Irish human genomes for the first time.
They discovered mass migrations to Ireland thousands of years ago resulted in huge changes to the ancient Irish genetic make-up.
A team of geneticists from Trinity College Dublin and archaeologists from Queen's University Belfast made the findings, which show a massive shift in our genetic mix over the course of just 1,000 years.
They believe the genetic influxes brought cultural change such as moving to settled farmsteads, bronze metalworking - and may have even been the origin of western Celtic language.
Researchers studied the genome of a woman farmer who lived 5,200 years ago near what is now Belfast. They also carried out DNA analysis of three men on Rathlin Island from 4,000 years ago in the Bronze Age after metalworking began.
The female farmer had an ancestry originating in the Middle East, where agriculture was invented. She had black hair and brown eyes, like current south Europeans.
The Bronze Age genomes of the men were different, with about a third of their ancestry coming from ancient sources by the Black Sea in modern-day Ukraine.
The three men's genomes showed significant difference, with one-third of their ancestry from the Pontic Steppe.
They had the most common Irish Y chromosome type, the blue eye gene variant.
There were also signs that they were lactose tolerant, and suffered from haemochromatosis - excessive iron retention - a disease commonly called the 'Celtic curse'.
The genomes of the woman farmer did not show these - highlighting that the genetic make-up of Irish people had changed dramatically in just 1,000 years.
Professor of population genetics in Trinity College Dublin Dan Bradley, who led the study, said a genome was the sum of all our genetic material.
"Every genome comes from two parents, four grandparents, eight great-grandparents and so on -it's a sample, not only of a person but of a community. You can use that to figure out how the community they come from related to other communities. You're building up a picture of how they relate to us today," he said.
"This degree of genetic change invites the possibility of other associated changes, perhaps even the introduction of language ancestral to western Celtic tongues," Professor Bradley added.
The landmark results are published today in international journal 'Proceedings of the National Academy of Sciences, USA'.