If you've got the room for it, the best thing to do with an oversize sing circle is to split it like an amoeba. Or rather, let it happen spontaneously.

At the Fleadh Ceoil in Clonmel a couple of years that happened to such an extent that I think it ended up with three or more song circles spread all over the upstairs assembly rooms and staircases in a hotel, with all instrumentalists playing in sessions downstairs out of the way of the singing.