As I understand the issue of performing copyright protected material, any event that has the possibility of having this happen is supposed to obtain license from the appropriate sources.

On the RenFaire circuit, this gets a bit sticky... Most small faires I'm familiar with have nothing but *wink, nudge*traditional or *wink wink* unattributable originals performed at them, and get away without the license.

I'd be surprised if a three weekend event didn't have a license for non-PD material... especially if they're booking multiple musical acts.

And what MMario said is correct... if Wild Mountain Thyme is a regular in the circuit for your neck of the woods, and the song is a 'tag' song of theirs, and especially if they're performing at the same event, it's a considerable rennie faux pas to play it at the event without their blessings. Consider it an unofficial kharmic greasing...

M