Anywhere that's big enough. Village and church halls are well liked by Bands because the acoustics are usually better than the average modern school hall, and the floor's often wooden and easier to dance on. Schools are sometimes warmer - an advantage in winter. Check out the costs and times available and don't forget to formally book the venue. Check out if it is Licensed for music, etc – if not you may have to speak to your local authority about Temporary Event Notice well in advance (but don't worry if you do, these are inexpensive and easy to arrange).
The room will need enough space for dancing. One "square" set of 8 dancers will take floor area of about 8 feet by 8 feet minimum. Hopefully, there'll be space for all of those attending to dance at once, and a seat for everyone to sit down at once to get their breath back. Seats around the dance floor are best - the nearer people are to the dance floor, the better atmosphere you’ll get. If they’re in another room, the Caller will have problems getting enough people up for each dance.
You don't need a stage for musicians, but it's great if you do. In either case, leave plenty of room for the band and clear their area completely. The Band will provide amplification and let you borrow it for the raffle draw and other announcements. They'll need at least one 13 amp plug within easy reach, good lighting, and their playing area to be completely dry (of rainwater, that is).
Ceilidh dancing can be done almost anywhere. Make sure there are no obstructions - loose carpets need sticking down with gaffer tape, and dusty barns need sweeping out at least twice to save bronchial attacks. Even a rucked-up mat or unlevel floor could give you a nasty claim for personal injury damages.Thick carpets and/or central heating make dancing feel a bit like swimming in treacle, but even then it's still possible.
If you're thinking of holding the dance outside, or in a marquee or a barn of any sort, then you've got the temperature to consider if it’s going to be draughty, particularly for those sitting-out a dance. Even English summers can be cool outside after 9.15pm. Options include starting earlier, or hiring one or two of those gas turbine-looking space heater thingies to give the place a blast of heat before the dance and in the break. Or you might not need any of those.
The Band must be under cover whether your dance is inside or out. A sprinkle of rain could make their amplifiers and microphones go up in smoke, their expensive instruments to warp and their hair go rusty.