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—the mortal Sons of Míl Espáine who, like many other early invaders of Ireland, came from Iberia. Geoffrey Keating, an Irish historian of the late 17th century, equates Iberia with the Land of the Dead.
In folk belief and practice, the aos sí are often appeased with offerings, and care is taken to avoid angering or insulting them. Often they are not named directly, but rather spoken of as "The Good Neighbors", "The Fair Folk", or simply "The Folk". The most common names for them, aos sí, aes sídhe, daoine sídhe (singular duine sídhe) and daoine sìth mean, literally, "people of the mounds" (referring to the sidhe). The aos sí are generally described as stunningly beautiful, though they can also be terrible and hideous.
Aos sí are sometimes seen as fierce guardians of their abodes—whether a fairy hill, a fairy ring, a special tree (often a hawthorn) or a particular loch or wood. The Gaelic Otherworld is seen as closer at the times of dusk and dawn, therefore this is a special time to the aos sí, as are some festivals such as Samhain, Beltane and Midsummer.
The sídhe are the hills or tumuli that dot the Irish landscape. In modern Irish the word is sí; in Scottish Gaelic, sìth; in Old Irish síde and the singular is síd. In a number of later, English-language texts, the word sídhe is incorrectly 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.