I was involved in trying to put together a book of Northumbrian pipe tunes about a year ago.
Unfortunately, for various reasons, the project didn't come to anything.
I did however spend some trying to work out the legality of publishing various tunes. The law is fairly confused, but as far as I could understand it - 'traditional' tunes are by their nature public domain - any arrangements, sheet music copies, midi files, abc files are the property of the author.
If the way you present them is your own work, then you should be fine. If you simply copy other people's presentation then you're breaking copyright law.
Having said that your chances of being found out or prosecuted are somewhat slim. You'll have to make a moral judgement.
Hope this helps