I know virtually nothing about music theory, but here are a couple of common examples of modal songs which have been "corrected" to modern sensibilities.

First - Greensleeves. The two alternate versions of the melody, the older one goes up a semitone on "love" (Alas, my love"), while the more modern one goes up a full tone.

Second - Mist-Covered Mountains. The version I play on harp has an F#, but I've heard my husband play it on the pipes with an F nat. Also, the tune "Hush, hush" is the same tune, also with the F natural.

I agree with an earlier reply to this thread about the harmonies of modal melodies - I have a number of Irish tune books where the accompanying chord is marked with a slash through it. This indicates that the third is not to be used.

Cindy Ellen