Depends on the dimensions of the mirror, wether it's framed and any fixing points built into the mirror/frame.
I'd be inclined to screw some batten to the wall with some Gripfill or no more nails style adhesive for extra bond. Then fix the mirror to that. It will be proud from the wall but won't go anywhere.
dimentions = big fuck off heavy mirror :-)
its wooden framed with 2 fixing points.
I thought about the batten route but it wouldn't look right.