Yes, Planxty was THE band back in their heyday.

About "Little Musgrave", it's odd that they found the words, with no tune, and had to find a tune to sing those words to, on their own, because that ballad is one of the most common "Child Ballads" with hundreds of different versions from various periods from England, Scotland, Ireland, and America.

If they'd looked around they would have found plenty of versions with words and the traditional tunes which went with them.

Joan Baez, around 1960, recorded a version sounding as if it were straight out of the English Middle Ages called "Matty Groves". (Musgrave is probably a corruption of Matty Groves.) In this version, there's a Lord living in a Castle with a page etc etc.

Then there are Appalachian versions where the Lord has become a farmer, his castle a barn, and Matty Groves, rather than being slain by the Lord's sword, is impaled on the farmer's pitchfork.

But the basic story is the same.