You should be able to remove the mirror without taking the sheet rock down. After that you have to determine what's next. The surface paper will likely peel off where the glue is or there will be big globs of glue on the surface. It might be repairable or it might easier/cheaper to rip it out and redo it. Or maybe just install another layer over top.
Where leather gloves, safety goggles and safety shoes when removing the mirror.
Where leather gloves, safety goggles and safety shoes when removing the mirror.