Aos Sí (people of the fairy mounds, also spelled Aes Sídhe) is the comprehensive term for the supernatural inhabitants of the Irish otherworld — a race of beautiful, powerful, and sometimes dangerous beings who live in the hollow hills and ancient sacred sites. They are identified in mythological texts with the Tuath Dé, the divine race of pre-Christian Ireland who were defeated in battle and withdrew into the sídhe. For poets, the Aos Sí represent the continued presence of the mythological past within the physical landscape, a world of heightened beauty and danger coexisting with the human world. Revival poetry is saturated with their presence.

The aos sí (people of the mounds) are central to Irish supernatural lore, representing the pre-Christian divine race who retreated underground. In poetry, they are ambiguous figures, capable of both benevolence and malice, and often serve as agents of transformation or tests for human protagonists. Understanding the aos sí requires recognizing their connection to specific landscapes (mounds, hills, lakes) and their role in maintaining the moral and spiritual balance of the world. They are not mere fairies but powerful, ancient beings whose respect must be earned.