"Wikipedia states:-
"The tune is that of an old Welsh air, first found in a musical manuscript by Welsh harpist John Parry Ddall (c. 1710–1782), but undoubtedly much older than that."

I believe 'dhall' means 'blind,' and the finder was John Parry. I have a book of his music, and the first sentence says, "It is thought that John Parry was born in 1710..."

Parry's tune is 'Nos Galan,' the minor tune. I have played a lot of early music, and I don't think you can claim that it is 'undoubtedly much older than that.' I have no trouble thinking it an 18th-C piece.

The Welsh harper Robin Huw Bowen publishes pocket tune books dedicated to harpers of yore, and I got the book from him.