There is a great book by Ossian Ellis, based on his analysis of the Ap Huw Penllin manuscript of Harp Tablature. I can't remember the title but I'll try and locate it for you.
Basically he has concluded that the harps were tuned in various modes, most Pentatonic and the melodies improvised over a pattern of 2 chords. Some of it is very like Piobroch, but that's another story.
All the best
PP