You are absolutely right Midchuck. If a song is in the public domain then anyone can sing it and if someone sings a song you were going to sing well, it's just tough luck.

However there is a convention in UK folk clubs that if a particular person is known to like a song and sings it on a regular basis, then it is considered discourteous for someone else in the club to sing that song. Of course it is only a convention and if you also like the song then you are perfectly free to sing it if you wish.