Danish/Norse is just as likely in principle, but the words don't have that feel to me; mind you, I'm far from being an expert.  We'll probably never know for sure.  The rhythm was as noted, but likely enough other singers would regularize it more.  I always feel a bit guilty about doubting MacColl's word on these things, but it does seem odd that his immediate family seems to have had so many rare versions of songs that nobody else had come across in such a long time.  Maybe I'm just a jaded cynic...