It was the major migration to Ireland 4,000 years ago (Bronze Age), which started in Eastern Europe that brought the R1b-L21 subclade Y-haplogroup, as well people with typical Irish features such as piercing blue eyes, Celtic complexion (pale or freckled skin) changed Ireland forever. The Irish are closely related to the Welsh and Scottish, according to a very recent study released by geneticists from Trinity College Dublin (Ireland) and archaeologists from The Queen's University Belfast (Northern Ireland). Before that most Irish were dark/black-haired, brown-eyed as the majority of Southern Europeans are. So literally blue eyes became the norm in Ireland, 4000 years ago only.