Sidhe are the more modern versions of the Tuatha Dé Danann, the fairy race of Old Ireland who were great masters of magic and appeared in early Celtic mythic tales such as Tochmarc Étaíne. After being conquered by the Sons of Mil (ancestors of the Irish people), the Tuatha Dé Danann retreated underground and dwindled into the still unearthly beautiful (but diminished) sidhe. The word “sidhe” originally referred to the fairy mounds where these beings lived. Tad Williams’s Sithi race from his epic fantasy trilogy Memory, Sorrow, and Thorn, is akin to the sidhe.
In folk belief and practice, the sidhe 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 sídhe). The sidhe are generally described as stunningly beautiful, though they can also be terrible and hideous.
Sidhe are 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. It is believed that infringing on these spaces will cause the sidhe to retaliate in an effort to remove the people or objects that invaded their homes. Many of these tales contribute to the changeling myth in west European folklore, with the sidhe kidnapping trespassers or replacing their children with changelings as a punishment for transgressing.
The sidhe are often connected to certain times of year and hours; as the Gaelic Otherworld is believed to come closer to the mortal world at the times of dusk and dawn, the sidhe correspondingly become easier to encounter. Some festivals such as Samhain, Beltane and Midsummer are also associated with the sidhe.