Hanging large mirrors on thin plaster panels

[Psyclist]

On Friday, my boss asked me to hang a large painting and two large mirrors to a wall in one of the rooms in the original hall. Now the hall is a John Dobson building and was built in 1818, however that doesn't mean the internal decorating was done in 1818 as I am told the building was stood in derelict condition for a long time.

In the room there are three large decorative panels on the wall, one smaller one above the fire on which I have hung the painting, and either side are two larger panels on which I wanted to hang the mirrors. Only once measured up, I found that the plaster panels seemed to have about an 1" to 1.5" gap behind it before what looks like plasterboard.
The surface plaster is incredibly brittle as I found out when our contracted joiner suggested I use a screw type plasterboard fitting which basically just burst a big hole in the wall (well, thumb size anyway...). But I can't seem to screw into anything to get any purchase. I tried screwing a 5" screw into the wall and it just disappeared straight through the wall.
Tried using a detector which picks up nothing apparently and can't seem to find any timbers to screw into.

Can anyone suggest any way of getting these large mirrors onto the wall securely?

Is there a way of adding a brace to the wall behind it or just generally strengthening the wall in order to hang heavy mirrors on it?

 

[foxhole]

Frame fixings.

 

[Psyclist]

Could you elaborate please?

 

[foxhole]

With a frame fixing the plug is the same length as the screw so you can normally get a good fixing even thru voids as long as the holes are not oversized for the fixing and they come in long lengths.