It has long been told that the Sidhe, the Irish fair folk, live under the hills. There are many hills known as fairy hills in the Brittish Isles, and there are also many rings of standing stones. You can get the facts through Wikipedia:
As part of the terms of their surrender to the Milesians, the Tuatha Dé Danann agreed to retreat and dwell underground in the sídhe (modern Irish: sí; Scottish Gaelic: sìth; Old Irish síde, singular síd), the hills or earthen mounds that dot the Irish landscape. In some later poetry, each tribe of the Tuatha Dé Danann was given its own mound.
In a number of later English language texts, the word sídhe is used both for the mounds and the people of the mounds. However sidh in older texts refers specifically to “the palaces, courts, halls or residences” of the ghostly beings that, according to Gaedhelic mythology, inhabit them.
The fact that many of these sídhe have been found to be ancient burial mounds, has contributed to the theory that the aos sí were the pre-Celtic occupants of Ireland. “The Book of Invasions”, “The Annals of the Four Masters”, and oral history support this view.
The story of the Aes Sídhe is found all over Scotland and Ireland, many tales referring to how the Norse invaders drove Scottish inhabitants underground to live in the hills. This part of the legend contributes to the Changeling myth in west European folklore.
But it is the stories and the tales, of sleeping on the mounds to learn magic, or of musicians stolen away under the hill to play at a fairy wedding, that I find so enchanting. In 1850, a farmer in the Orkney Isles uncovered the true fairy mounds. There is truth to be found in fairy tales, and we are discovering it every day.