Mick,
The format usually works quite well. When the players don't know where you are are going they usually just watch and enjoy. They may join in when they figure it out.
I , also sometimes sing without music. I don't do a lot of material in Gaelic but when I do the tradition is a capella. Also I have written a couple of un-acompanied songs that I sing from time to time.
         Slainte,
               Sandy