My Scottish hymnal (1939) gives the same attribution as the above - it describes the tune (Slane) as an Irish Traditional Melody and says the words are "ancient Irish, trans. Mary Byrne, versified by Eleanor Hull."

What we don't really know - at least I don't - is whether those words (Be Thou My Vision) became linked with that tune only during the 20th century when the hymn books were compiled, or whether they'd been associated together for longer than that. As far as I know, lyrics of non-Biblical origin were not sung as hymns in church until about the 19th century, but I think the Irish Gaelic tradition may include songs of a meditative/religious nature that date back for many centuries.

And it seems likely that the Banks of the Bann words are probably no older than the hymn words, possibly a lot younger.

It's not such a simple question as it first appears, is it?

Marje