Good definitions of ceilidh above. Here are the OED comments. The word was not admitted to its hallowed pages until the 1987 supplement.

Ceilidh, also ceilidhe- Old Irish céile, companion [also ref. to Sc. word, etc.].
a. An evening visit, a friendly social call. b. A session of traditional music, storytelling or dancing.
1875, from Celtic Magazine. "The fire in the center of the room was almost a necessity in the good old ceilidh days."
1904, Daily Chronicle. "Participants narrated their incidents at the 'ceilidh' around the cottage fire."
In 1959, the sense of a party or hootnanny "all over the British Isles" at the pub, club or private house featuring folkmusic appears in print.

Esentially the same definitions are in Merriam Webster's Third International Dictionary, but the word is left out of the Collegiate edition.