The archaeological record of humans in Ireland begins in Castlepook Cave, County Clare, near the coastal city of Cork on the south coast. Here, a reindeer’s femur has been discovered that radiocarbon dating has identified as 33,000 years old. Initially excavated over a hundred years ago by the naturalist Richard Usher from a place he called Mammoth Cave (due to the abundance of mammoth bones found there) between 1904 and 1912, the reindeer bone was recently reexamined with modern techniques and technology and found to possess clear signs of butchery. It is the earliest specimen indicating human habitation in Ireland. But it doesn’t prove continuous habitation. Sea levels have changed dramatically over the eons, at times exposing land bridges to the British Isles that early migrants may have crossed only seasonally. Others might have been able to access the Irish shorelines in boats when the seas were far more shallow. Later immigrants almost certainly came in waves from the north and east and south, adding layer upon layer to the societies and cultures that already existed there.

The earliest entries for human specimens in Ireland currently listed at haplogroup.info are two DNA samples—one a Mesolithic specimen from a cave in Limerick whose U5 haplotype roughly corresponds to the Gravettian Culture and whose genetics indicate he may have had black skin, brown hair, and blue eyes. And another Gravettian-era sample that was found near Galway, a more ancient relative of Cheddar Man, the famous specimen from Somerset, England from 9000 years ago.