An interesting read is Haunted Land by Paul Devereux. Over 2,000 ring-forts dot areas of the Irish landscape which now contain thorn bushes. This site type originated in the Iron Age and date between AD 500 and 1200. To generations of country people these forts were seen as one of the places where fairies dwelt, living underground and from which they emerged at night or at special times to travel across the countryside. I think it would be astonishing to visit one of these ares.