The venerable Jim Maclean is right, and Led Zepplin is no comparison.. though they themselves have faced the issue over plagiarism and their nicking some parts of 'Stairway to heaven'. The difference that has gone over your head, is that lyrics from a couple of hundred years ago have no copyright.. but a modern arrangement of the song can have a copyright if you add your own stamp to them, creating your own arrangement.   If the tune is original as well then this adds even more weight to the fact it holds a legal copyright. So anybody who performs Wild Mountain Thyme as trad. in instrumental form is using the McPeake copyright work, unless it is proved different.   There won't be too many years before it does become totally trad. 70 years after the death of the author/composer. Copyright in a work only exists from the date of the first fixed recording, or writing down of the work, and that lasts for 70 years post mortem of authorship. Even if McPeake had heard it from an uncle (which he ambiguously says in the 1952 field recording), copyright would still belong to the 1950's McPeake version, as you'd have to find it in fixed form pre-dating that. Back to the Led Zepplin analogy.. the difference is that the song you'd be tampering with would be a modern copyright work. Not one from hundreds of years ago which has been substantially re-written and given a new tune thereby creating a new lyrical arrangement and in terms of tune a copyright work.