Mounting something to stud-less plaster exterior wall

Sounds like plaster walls and the ceiling is gypsum boards covered with just a skim coat of plaster. My house in the midwest US has a similar setup. It was during a transition period (40s-50s) from lathes to gypsum boards so the walls have lathe and then rough and finish plaster but the ceilings have gypsum boards covered with a thin finish plaster.

To hang pictures and stuff, I typically start the hole with a drill to get through the harder plaster layer and then tap the nail in the rest of the way. For heavy shelves in the closets, I've drilled holes and used those plastic inserts you tap in with a hammer and then put a screw into.

The blisters in the photo are due to moisture (water vapor) being forced out of the from the opposite side or from the ground. Moisture permeates through the plaster in the path of least resistance, such as a fissure or hairline cracks. If the surface has been painted the water will actually blister the paint as it continues through the wall. The powdery residue you notice is a combination of minerals left behind when the moisture evaporated and whatever effluence it picked up traveling through the plaster. To solve the problem first eliminate the cause of the moisture. This could be any thing from a wayward sprinkler that splashes water against the walls surface to gutters saturating the ground that is against the wall. If the wall is in contact with the soil, excavate to the footing clean the surface and when dry apply a concrete sealer like Dryloc. To repair the blisters wire brush the loose plaster scrape any flaking paint, apply a concrete adhesive to the surface (not critical to the repair, but it helps), and apply a plaster patch as needed. It will blister in another location if the water source isn't corrected.