Sidhe (Aos Sí / Aos Sídhe)

The term derives from Old Irish sidhe meaning 'hill' or 'mound' and the compound aos sí / aos sídhe literally frames them as the 'people of the mounds.' In later folkloric usage the Sidhe are identified with or said to descend from the Tuatha Dé Danann — the mythic pre-Christian gods or hero-people of Irish tradition — so that the Sidhe function as a continuity between the old deities and later local fairy belief.

Sources do not give a single fixed appearance. Traditional and scholarly summaries treat the Sidhe as a supernatural race comparable to 'elves' (implying humanoid otherworldliness), while some accounts equate them with the Tuatha Dé Danann and therefore allow for imagining them as powerful, sometimes beautiful or noble beings. Specific named types within the category (for example the leanan sídhe as a beautiful fairy woman, or the cat sídhe/Cat sith as a fairy cat) carry their own descriptive traits in folklore.

Folkloric and interpretive sources attribute longevity, an existence in an overlapping unseen realm, and occasional returns to the physical world for short periods. As descendants or equivalents of the Tuatha Dé Danann they are presented as superhuman in power and influence. Behavior is ambivalent: Sidhe can inspire or aid humans (the leanan sídhe inspiring artists) but also harm or take from mortals; they may be territorial and interact unpredictably with people according to local belief.