Cats are attracted to the warmth of the fire and to anything that flickers. Fireplaces must be screened off with a fireproof screen. If your cat persists, use a can of compressed air as a deterrent. There are special cans for pets, but the type you use to clean a computer keyboard will work just as well. A water spray bottle is another good alternative. Eventually, if you even leave the can or the water bottle in the vicinity of the fireplace (not too close to the fire for safety reasons), she won't even attempt to go close.
No cat is ever safe around candles, whether they are placed on a low table or up on a high mantelpiece. Cats easily can jump up to investigate. Again, you can try using a can of compressed air or a water bottle as training deterrent not to go close, but to be really safe, purchase flameless candles instead. That way you will have both the ambience and peace of mind.