Aos Sí

Medieval literature often equates the aes síde or fír síde with the Tuatha Dé Danann, interpreting the Aos Sí as the later literary or folkloric forms of those older pre-Christian deities; other traditions describe them as nature spirits, ancestral or familial spirits, or in some accounts as fallen angels not damned. Their identity is tied to their dwelling-place (the sídhe, i.e. mounds or hillocks), and sources present multiple overlapping origin notions rather than a single canonical provenance.

Accounts are variable and general: the Aos Sí are commonly said to be stunningly beautiful but can also be terrible or hideous. Specific named subtypes carry particular forms (for example cat-sìth as a fairy cat, cù-sìth as a fairy dog), but the supplied sources do not provide consistent anatomical detail. Linguistically, older Irish usage distinguishes the sídhe as the residences (halls/mounds) and later English usage sometimes conflates the dwellings with the people.

Aos Sí inhabit an Otherworld that interpenetrates the human landscape and interact with people in varied ways: they guard their abodes fiercely and may retaliate against those who infringe sacred places, can abduct trespassers and appear in changeling narratives (replacing or stealing children), and include subtypes with specialised roles (bean sídhe/banshee who keens to announce death; bean-nighe who washes the clothes of those about to die; leanan sídhe as a fairy lover; sluagh as an airborne host). Their activity is liminal and seasonal — they come nearer at dawn and dusk and around festival times — and moral ambivalence is a constant theme: they may help or harm depending on how they are treated.