
(Wikimedia CC, photo by Humphrey Bolton)
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The so-called “Wexford Carol,” which comes from Ireland’s County Wexford and which seems to date back to the twelfth century, may be among the oldest Christmas carols — and, thus, among the oldest pieces still performed — in the Western musical tradition:
https://www.youtube.com/watch?v=2b6URDdh_7A