what kind of wall?

|I am complete DIY novice and new home owner and I have a 30 lbs. mirror |I want to hang. I can't figure out what kind of walls I have. The house |is listed but remodeled in the 1980's. There is a hole drilled |completely through the interior wall between lounge and closet and |there is a grey-white chalking plaster all the way through - 5 inches. |Also, I have a used a stud/joist sensor and there doesn't seem to be |any joists. Any ideas? What are the best fixings to use?

It will be ?Thermalite? blocks. Not as strong as bricks, but fairly strong, use a slightly larger plastic plug than you would usually expect, say 8mm dia.