As an occassional folk club visitor I have some difficulty with the "that song is sung by so-and-so" mentality. If the club is open to outsiders/visitors then they must expect that there will be occassions when the same song that they've heard before by one of their members will crop up at the hands of a visitor.
Short of asking everybody present to submit a list of "intended songs" for the evening, I can't see how that can be avoided.
Personally I wouldn't want to sing a song that has been regularly performed at a particular club for fear of being boring, nor would I want to become known for singing a particular song, for the same reason.
With regard to Mrs. Ducks friend who wouldn't share the lyrics to a song I must say I find that attitude strange.
As a guitar player I've spent a lot of time and effort learning pieces of music and once in a while you come across the "trick" to a particular song, that could be in the form of a particular tuning, neck position or lick, that gives the song/music that special edge.
If I've ever been asked to show how I played something I could never adopt the attitude that, because I'd done all the work learning it, I wouldn't want to share that with anyone else.
True ownership belongs to the songwriter and he has every right to withhold permission to sing it or impart knowledge of it but I suspect most songwriters would adopt the attitude that McGrath subscribes to and be proud to have a song out there, in the public domain, that others want to sing.
If we all keep our "knowledge" under wraps then what musical tradition will there be for future generations.