In Irish folklore, the supernatural fairy race said to live in underground mounds is the aos sí.
The name aos sí means “people of the mounds” or “folk of the fairy mounds.” The mounds themselves are associated with ancient burial places and are imagined as entrances to the Otherworld. In English-language writing, the beings may also be called the fairy folk, fairies, or daoine sídhe, while sídhe can refer more precisely to their supernatural residences or mound-world.
Many Gaelic traditions connect the aos sí with the Tuatha Dé Danann, the divine or semi-divine people of Irish mythology. In a well-known mythic pattern, the Tuatha Dé Danann retreat underground after the Milesians, ancestors of later Irish people, conquer Ireland. This story helped explain why ancient earthworks and burial mounds were treated as homes of an older supernatural population.
The aos sí are not simply tiny, harmless fairies. Folklore portrays them as beautiful, powerful, and potentially dangerous guardians of mounds, thorn trees, lakes, and other special places. People traditionally avoided disturbing these sites and used respectful euphemisms such as “the Good Neighbours” to avoid offending them.