Getting back to the OP's point about the use of harmony by bands, I think it's worth pointing out that Irish and English fiddling have quite divergent histories. Both may or may not have used harmony at different times, but sadly there is no continuity in either tradition to modern styles.
In Ireland, traditional music was rather frowned on by the late 19th century- the tunes were "patriotic", but a "refined" style, with genteel piano accompaniment was far more socially acceptable. The traditional style was preserved via American music hall and recordings (Tuohy, Coleman etc), and largely re- invented in Ireland and England post WW2. The recordings were mostly solo, or with (often bad) piano accompaniment, and the session style that developed wasn't conducive to harmonisation.
English traditional dance music may have been harmonised up to the 19th century, as many musicians doubled as west gallery players, but this fell out of favour in the 19th century, replaced by the organ and the Oxford Movement. As far as I know, there are no early recordings of dance music played in this style (love to be proved wrong). Early recordings were of soloists, and perhaps the collectors looked for a "rougher" and plainer style as more authentic. I don't think Georgina Boyes addressed dance music- did anyone else?- but active selection by collectors certainly affected the preservation of both song and dance.
English trad dance music was in a pretty bad way by the time the EFDSS climbed down from its pulpit. Again the classically- influenced style with tinkly piano and correctly- dressed gentility was preferred. It was only from the 60s onwards that it started to relax its sphincter. The slower pace of English music (Irish had rather lost its connection to dance- not much room to do it in smoky crowded London pubs) and the fact that it's more dance and workshop based and less session based is much more encouraging of harmony. It's a new take on the tradition. That's not to say it's "bad", "wrong", "inauthentic" or anything like that.
Living traditions reconfigure themselves as their practitioners change and develop, and both Irish and English traditions have been almost brought back from the dead. Both are now healthy- long may it remain so.