Many Irish laments are embellished with the use of minor tones 'Glory-oh, glory-oh to the bold Fenian men', 'Revenge for Skibbereen'. Even within hornpipes and reels one can hear minor keys. I like all kinds of musical genre but I think that the reason for "Folk Music (being) a bit lacking in harmonic variety -- generally no minor keys" is due to poor musicianship.

I love adding minors where appropriate (no need to over do it) but my instrument (small accordion)is limited to major bass notes. But I hold the bass fundamental when playing the minor melody and there is no conflict in tonality.

My twopence worth

Regards
Padraig
Ireland