The aos sí are a supernatural race in Irish mythology and Scottish mythology comparable to the fairies or elves.
They are said to live underground in the fairy mounds, across the western sea, or in an invisible world that coexists with the world of humans. This world is described as a parallel universe in which the aos sí walk amongst the living. In the Irish language, aos sí means “people of the mounds” (the mounds are known in Irish as “the sídhe”).
In many Gaelic tales the aos sí are later, literary versions of the Tuatha Dé Danann (“People of the Goddess Danu”) – the deities and deified ancestors of Irish mythology. Some sources describe them as the survivors of the Tuatha Dé Danann who retreated into the Otherworld after they were defeated by the Milesians.