Scientists in the UK, Ireland and the US analysed the teeth and skeletons of 17 brown bears found at eight caves in Ireland.
And mitochondrial DNA showed the now extinct Irish brown bears were the ancestors of all modern polar bears.
Dr Ceiridwen Edwards one of the leading researchers said: “It's amazing to think that Irish brown bears are the ancestors of the modern maternal polar bear lineage.
“As the hybridisation between the two species occurred at a time when their home ranges overlapped, most likely during environmental stress, this has implications for polar bears in today's climate.”
Previously it was thought modern polar bears originated in Alaska.