We are doing this arrangement for Christmas. I had not heard it before, that I remember.
I was disappointed that it isn't actually 12th C and the oldest current Christmas carol, as I was told. It is traditional Irish, but newer, possibly only 18th C and certainly not further back than 16th. It does have a Celtic sound, and a bodhran is often used to make it sound older. The problem with that is that the bodhran isn't all that old, either, probably only late 18th C. Frame hand drums have been used for centuries, so it isn't that far off to imagine that something like it was used in more ancient times.
The lullaby in "Frozen II," a modern composition, has a similar sound, and I told the granddaughters to listen and compare. That song is also meant to suggest a tune and poem of great age with obscure meaning, being handed down mother-to-child, so the composer got the feeling of it correct. It's quite a contrast with Kristoff's Song, which sounds a lot like Michael W. Smith's "Place In This World."