I don't believe there is an answer to the question of what songs are offensive, in isolation from the singer's relationship with the audience. If you have served your time with an audience and they trust you, you can sing anything you want. I have sung Trelawney in an Irish Nationalist pub, and I've heard someone else there do The Sash. But if they don't know you, you can cause as much, if not more, offense, by singing even a song they DO agree with the content of. Anyone who walks in off the street and starts singing controversial songs deserves what they get.