As I understand it, Mingulay Boat Song has its origins way back and was recontructed from the Gaelic to give us waht we know now.

Many of Les Sullivan's songs have been referred to as trad by those who have only heard them second hand and not known the true source, and as above posts indicate this is not unusual.

All I can say is, if something you wrote confuses people in this way then good on you. It proves you are doing something right. It has only ever happened to me once, I sang a song I had written in the Middle Bar and someone came up to me afterwards saying that it was a piece of Burns he had never heard before. My reply was (when the astonishment subsided) that the Rabbi only wrote it in spirit and not in fact. Nice moment though.