John M., I may be beating a dead horse, but the idea of intellectual-property law is that the person who wrote the song owns it. You may think you're doing Christy Moore a favor, but it's not your decision; it's his.

Maybe you don't like that philosophy, but if not, as Chet asks, what's the alternative? (A sliding scale?)

What if someone in the audience doesn't like your performance (since there's no accounting for taste)? Your advertisement wasn't worth much then.

If you think you should get a cut, nothing prevents you from writing to Christy Moore and asking for it. You could even ask him to reimburse you if BMI or ASCAP manages to collect money from you--as well as asking him for 10% of his royalties from sales in your town, on the theory that your advertising was responsible. See what he says.

I'll bet a nickel the most he'd do is let you perform his songs royalty-free. If he actually wanted you to advertise, I'll bet he'd want to hear a demo instead of letting you decide that you're good advertising. And I suspect (not knowing anything about him) that he might say he makes his living from music and he wants all the money he's entitled to.