"the hills of the fairies," 1793; but in Yeats, "the fairie folk" (1899), and in his use an ellipsis of Irish (aos) sidhe "people of the faerie mound" (compare second element in banshee).
Entries linking to sidhe
banshee n.
in Irish folklore, a type of female fairy believed to foretell deaths by singing in a mournful, unearthly voice, 1771, from phonetic spelling of Irish bean sidhe "female of the Elves," from bean "woman" (from PIE root *gwen- "woman") + Irish sidhe (Gaelic sith) "fairy" or sid "fairy mound" (from PIE root *sed- (1) "to sit"). Sidhe sometimes is confused with sithe, genitive of sith "peace."