Fireplace switches are typically a low volt loop aren't they? You would have to pull out the switch and see if it is connected to hi volt or low volt wires. If hi volt an X10 non dimming relay should work but you'd need a neutral in the box (white wire) for the drain current of the relay I think. If low volt and near AC you'd have to dangle a dongle inside the wall (and know what you are doing).