...there are surprisingly few mermaid songs...

There's the Great Silkie of Sule Skerry of course.

I agree with Celtaddict - whether Catherine turns up again or not is beside the point. There's an interesting song out there waiting to be tracked down.

Looking at that snatch of the song it occured to me that the structure looks as if it might possibly be French - that repetition of the couplet with the refrain in between, I mean. And there are, or at least were, French speaking communities in Newfoundland. (And the French islands of St Pierre et Miquelon are only next door).