Hanging a mirror on unknow wall type
In the case of either plaster or Thermalite panels, plastic plug-type anchors should suffice. I'd use those sized for a #12 or #14 screw of adequate length (per the anchor manufacturer's suggestion), and be sure to drill exactly the correct size. 

Be sure that you only leave the screw protruding only as far as necessary to allow the hooks to engage. Torsional force is what tears anchors out of walls, so you want to keep the load tight against the wall.