There are a few things to look at - sounds to me like you are dealing with horse hair plaster? not the more modern.
Ok, there are some metal washers - about 3.4" in diamater, with a hole in the center that will accomodate a standard dry wall screw - usually 1 5/8" is a standard, you can get longer ones depending on the need. Get a bunch of these washers, check building supply or hardware stores to start, depending on how many "bubbles you have.
Next, gently push against them and try to find the "hard spots" - ie. places were there is not much give, you can "souround" the bubble with these washers and screws and pull the plaster tight to the lath strips - the washers usually flatten out when enough pressure is applied, but they will keep you from pulling the small screw head through the plaster - which will do you no good.
You can also try to pull the bubble tight with these screws and see what happens. Some times this works, but I have found that usually the plaster just crumbles out, what has essentially happened here is that the plaster "grabbers" have seperated from the plaster itself, that is, when you apply the plaster to a lath strip the strips have space and the plaster curls around behind them and holds the surface plaster to the wall - when this seperates the blisters occur.
In this case, when you can not pull the plaster back to the lath strip, you can take it out altogether. If it is a large area you may want to learn a bit about plastering, but for a small area you can consider trying "Duro-bond" this comes in bags and has different setting times - for example "Duro-bond, 45" means that it has 45 min. working time. This sets harder than regular joint compound or spakeling and you can mix it yourself - so you can control the consistency - you want something like a heavy cream, as you want to force it into the openings in the lath strips, have it "grab" the back side and set - not run down the wall.
You can build up layers as you see fit untill you match the sourounding surface - come close - I like to apply a layer of fiber glass tape to the patch before I put on the final coats, be sure to extend from the patch out over the old plaster by at least 2 inches. You can also hit the final coat with joint coumpound which may be easier to work and may sand out better, it does take more time to dry and will shrink and crack if applied to heavy at one time. Good luck.
The suggestions to make a good inspection of the walls before covering them is excellent - now is the time. Also cleaning out the cracks, and fiber glass taping over them is good - you could also use the Duro bond here as the base coat.
