There's no such thing as "right" you know, cap'n sorr. You've had two cracks at it, but I think you'll find that Pontchartrain also has its adherents as the "right" spelling, so there's three ways for a start. And, as it is indeed a traditional song, you can't really object if Christy Moore's version is the "lakes" (plural). That is how it is sung, those are the words. If you want to sing it in the singular, go ahead. The world of traditional song is Liberty Hall.
   I think you might find that there is now one lake, but there used to be a few(like Tarn Howes in the lake district, which used to be called "The Tarns" when there were three, but a dam caused a certain amount of amalgamation).
    And as regards Chrity M: I knew what he meant by "traditional", so communication was possible. The English language is defined by usage, that's the way it is. There are no legal defintions(except in law, and that varies too!). Dictionaries can provide guidance, but only guidance.