What you do with a song depends on what you see it as.

If you see "Sammy's Bar" as a Cyril Tawney piece, you sing it the way he wrote it. If you see it as his maritime interpretation of the genre of "St. James Infirmary," you might fiddle the words a little to meet your own needs.

It's called the folk process. As long as you credit the originator and say it's your interpretation, no one can really fault you. If the piece is trad, you have even more latitude.