So far as I can tell, "Jubal" occurs in one example only of the song, noted by Ralph Vaughan Williams from a Stephen Pole in Norfolk. The Young Tradition recorded an arrangement of it, and I expect a lot of people learned it that way (Brass Monkey recorded it as well, though much later).

In other traditional versions where that form of words occurs (Copper Family, for instance), it's usually sung "joyful". That may be all there is to it.