What's behind your plaster is not strapping, but wood lathe. Lathe and plaster are not separate items, but integral parts of your wall.

When plaster is installed it is sort of a stiffish glop, and it is pressed against the underlying lathe layer; some of the plaster is smushed through the openings between the lathe rows. When the plaster dries those pushed-through parts contract and stick, hard, to the back of the lathe forming what are called "keys". If you can find the back of some plaster to look at you will see exactly what I mean. These "keys" are what hold the plaster to the wall; any damage to them and your plaster will begin to fail (crack, break into pieces, fall off or sag).

Fifteen pounds of mirror sounds a bit heavy to try to hang from just your plaster and lathe; the usual contemporary solution would be to install special bolts that go through the plaster and lathe and then press back against the back surface to support the item hanging on the front. Depending on the remaining strength of your wall this might work OK.

Unlike a sheetrock wall, lathe is supported by a sometimes irregular patterns of vertical supports, but there vertucals under there. I think the preferred traditional (and safest for your plaster) solution would be to install some type of hanging rail bridging between two vertical supports and lagged into them through the plaster. Then hang the mirrors from this rail. If you look at pictures of Victorian houses you can often see this type of rail which is called a picture rail and was devised to handle exactly the problem you have.

Good luck!
Molly~

I use those expanding bolts too. I'm not sure that is the name I want, they have a brace that folds, you push it into the wall on the end of a screw and tighten the screw to tighten it against the slats.

I usually put one at the top and one hidden behind the mirror on a separate wire as a safety.

Do yourself a favor, use 'snap toggle' toggle bolts we use them to hang 19' tv's and other heavy units off the wall at the hospital where I work............

Here is a link that might be useful: http://www.mechanicalplastics.com/technical_bulletins.html#toggle

Thank you for your input everyone! As extreme luck would have it, we ended up having studs every 16". It was a miracle. We have not been so fortunate in other areas of the house, so your input will benefit greatly when we tackle those rooms!

Thanks!