Reindeer were once common in Ireland
It is difficult to believe that herds of Reindeer once roamed the Irish countryside and were native breeding members of our mammal fauna. That was at the end of the last ice age, of course, some 12,000 years ago. The Reindeer were accompanied by Arctic Foxes, Norwegian Lemmings, and Giant Irish Deer. As the ice melted and the climate improved they all died out and became extinct and were replaced by newcomers like the Irish Hare, Irish Stoat, Brown Bear, Grey Wolf, and possibly Red Deer. It is believed that the new arrivals colonised Ireland from the ice-free southern part of England.
Source: independent.ie