Thanks, Peace, I have seen those sites. Any takers on the central question? Any Middle English scholars who could judge whether, in this context, there could be a play on words for Saint Bride, or Saint Brighid, or is this, as most suggest, a purely secular song? (I know we'll never know for sure, but I'd value opinions from those in the know more than I on such matters.)

It occurs to me as odd, in a way, that the monkish religion that saw women largely as either whore temptresses, destructive daughters of Eve, or as the impossible virgin mother, should use sexual attraction as a metaphor for yearning for the divine. On the other hand, plenty of scope for Freudian theories here.

Thinking further, in the book of the prophet Hosea, Israel's relationship with God is likened to that of a whoring wife. I suppose that this answers my question (unless you know otherwise): the metaphor of human sexual relations for the relationship with the divine was there way back in the Tenakh, so the 13th/14th century song could be rather late example of the genre, rather than an early one, as I first thought.