I've always had the feeling that it may have started off as a "Last Good Night" song, a song of someone who is due to be executed in the morning - and the words of that Scottish predecessor version seem to tie in with that. Especially that first couplet -

O this is my departing time! For here nae langer maun I stay:
There's no a friend or fae o' mine, but wishes that I were awa.