Les in Chorlton: you don't actually need as many as twenty-seven unamplified instruments to dance to. These tunes were made by fiddlers mostly, and they mostly worked on their own. Obviously a big hall or marquee full of people might need a bit more oomph, but I would suggest an accordion,2 fiddlers, cello/bass and a bit of percussion(or some similar lineup) is quite adequate for a standard village hall with 10-20 couples on the the floor.
This is not a theory by the way, this is actual practise. I have often played acoustically for dancing, as I am sure most musicians have.
I would put in a plea for the banjo here. Can't be beat for cutting through asnd giving a clear beat in an unamplified environment. The bodhran, however, is better used as a drinks tray for the musicians.The flubadubadub nose it makes when played doesn't always help in an unamplified lineup trying to hold a tight rhythm against background noise.