Celtic Ireland has evidence of human presence about 12,500 years ago The receding of the ice around 9700 BC (after the Younger Dryas cold phase of the Quaternary) heralds the beginning of Prehistoric Ireland, which includes the archaeological periods of the Mesolithic, the Neolithic from about 4000 BC, the Copper and Bronze Age from about 2300 BC and Iron Age beginning about 600 BC. Ireland's bronze age begins with the emergence of "protohistoric" Gaelic Ireland in the 2nd Millennium BC and ends with arrival of Celtic la Tène culture by central Europe By the late 4th century AD Christianity had begun to gradually subsume or replace the earlier Celtic polytheism. By the end of the 6th century it had introduced writing along with a predominantly monastic Celtic Christian church, profoundly altering Irish society [ https://www.ireland-now.com/history.html ]