At the start of the week of the Summer Solstice, many will once again marvel at the magnificent structures built across Ireland thousands of years ago, including Newgrange in Co Meath. The structures were built by the descendants of the first peoples that landed on the island in what is believed to be two migratory waves from around 10,000 years ago. Until recently it was argued that much of the population of Ireland was descended from this group of people, mainly from what is now Spain and southern France, with the arrival of the Celts around 500 BC adding what...