OK-here goes! I discussed this with my husband who is a Celtic musician and he explained that, while what I said earlier in this string was true, it was not complete. This is what a planxty is--to wit: A planxty is a tune written in return for a favor. Sometimes it is a simple thank you and sometimes it is a quid-pro-quo--a return for services rendered. This accounts for many planxtys having names attached. A musician might, for example, in return for a meal and lodging write a tune for his host or in praise of his host's lady. Or he might write one for a lady who did him a kindness. I hope this settles for you what a planxty is despite some of the weirder stabs at explanations I've just read in this string.