A surprising number of contributors to this thread perpetuate the ancient(well recent) myth that people played in unison till recording came along. The fact is we are are obviously rather short of evidence as to how people played before recording came along.
. I would draw your attention to the rather obvious fact that the most consistently common way of constructing the second part of a tune inthe Irish fiddle tradition is as a variant on the harmonic structure of the first part, and not as a variant on its melodic structure.( I analysed more than a 1000 tunes to come to this conclusion, I'm not just making a wild guess).
That suggests to me that playing in harmony, or with ccompaniment, is a fairly long-standing tradition( at last over the 2/300 years period that the tunes in question date from).