For starters, you need a good bra. Unless you succumb to sewing straps on it (and if you can get something that'll work, right colour and such, that might not be a bad idea, since you don't want to flash the ceilidh), that means strapless, so it's even more important that it's a good fit and good bra. You can then use special tape to stick the bra to the inside of the dress, although it might leave marks on the bra (still haven't got round to getting them off mine). How good a fit is the dress? If it has a tendency to fall down already, sewing straps on looks like an even better idea.

So what sort of green is it, what's the fabric like? Tell!