I've been in several Irish pubs where everyone is accoustic - but the pubs had small hanging mikes on arms so they could be moved around but still be fairly unobtrusive. Whoever was leading the song/ tune could just be that little bit louder than the rest. It seemed to work well.

I've also played in a scratch band at a festival where the sound man turned off one of the instruments - unbewnown to the musician. He kept on complaining about the monitor sound - so he got plenty of that - but nothing going out. I thought he was a good PA man.
FloraG.