I take it you have 12mm plasterboard dabbed onto thermalite blocks?
depending n how close to the edge of the mirror you have. Cut a bit of 18mm wood 3 inch square, chisel out a 3 inch square out of the plasterboard, attach wood to blockwork then attach mirror to wood. Hopefully it will all be hidden by the mirror.
The problem you have now in trying to attach the mirror directly to the blockwork is that any fixiig long enough will probably be too fat to go through hole in mirror.
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