Aos sí or aos sídhe is the Irish name for a supernatural race in Gaelic folklore, similar to elves. They are said to descend from the Tuatha Dé Danann or the gods of Irish mythology.
Riders of the Sidhe (1911), painting by John Duncan
Knockshee, County Down, known in Irish as Cnoc Sí ("hill of the fairy mound")
Hawthorn tree, considered in local Irish lore (and Celtic folklore in general) to be sacred to the aos sí
In Celtic mythology, the Otherworld is the realm of the deities and possibly also the dead. In Gaelic and Brittonic myth it is usually a supernatural realm of everlasting youth, beauty, health, abunda
Aos Sí
…gods of Irish mythology. The term aos sí means "folk of the sí"; these are the mounds in which they are said to dwell, which are seen as portals to an Otherworld. Such abodes are referred to in English as "shees", "fairy mounds", "elf mounds" or "hollow hills". The aos sí interact with humans and the human world…
The 'Land of the Ever Young' depicted by Arthur Rackham in Irish Fairy Tales (1920).
Oisín and Niamh approaching a palace in Tír na nÓg, illustration by Stephen Reid in T. W. Rolleston's The High Deeds of Finn (1910)