It depends on the ceilidh! The one my wife and I will be attending after the Clan Donnachaid Burns night supper in January is a formal affair, so black tie is required. One of my own personal ceilidhs, on the other hand... Remaining fully clothed is itself occasionally optional! As to the formal we are going to. I will be wearing a Prince Charlie, my Albannanach tartan kilt, a tuxedo shirt, purple Lewis Rich kilt kilt hose, a black bow tie, kilt pin, semi dress sporran, and ghillie brogues. For something advertised as formal, I would wear no less. So I would think it all depends on the advertised level of formality- at least, with this being in the US and all. In Scotland, I’m sure it goes without saying that the level of formality is as least semiformal, but in the States, we assume that everything is very casual unless stated otherwise. Were I in Scotland going to a Scottish celidh, I would assume it to be at least a semiformal affair.