I don't know if this would be helpful, but this is an outline of Irish Tradtional Music form, which is highly modal.
Written by a member:
Some way down is a section called Chord Structure, which describes each mode and its commonly associated chords, which may not be so obvious when you're used to classical harmony. (Let's not mention harmonic progressions as this can open hornets' nests among trad purists!) But maybe could be a starting point for arranging?
My still elemantary experience of playing Irish and medieval/renaissance tunes on the harp is that there is often a similar sound, the harmonies are more open and less altered than in later music, eg not so many 7ths etc. The ancient music of course often has wonderful crunchy dissonances.
Anyway, a good read I think.