Although insulation in the walls is very important in cutting down on heat loss, insulating the ceiling of of the heated space is much more important because heat rises. Do you have an attic, and is the floor of the attic insulated? That's where you should start. Make sure your attic insulation is up to current standards. Next make sure you put weather stripping whereever drafts can come in, and that you have either storm windows or EnergyStar rated windows. After you do these things, you may find that you do not need to insulate the walls.

Since you have already removed the plaster and lathe, the walls will be your number one priority.

First, repair and repoint the masonry wherever needed. Then apply a liquid vapour barrier to the inside of the masonry. Next, recommended insulation includes closed cell polyurethane spray (my favourite), expanded polystyrene board (XPS), or extruded polystyrene (EPS) depending on your target R factor, the space you have to work with or other variables.

Pay special attention to sealing the old window frames to the new insulation, even building new internal frames if necessary.

Your goal is to make the insulation as tight and free from air movement as possible.

Read the up-to-date digest in the link below from one of the leading experts in the field for details.

Once you're done, get to work on the other areas of your home.

Here is a link that might be useful: . Building Science Digest 114

I'd also refer you to the historic preservation brief on insulation. It specifically addresses insulation needs of all types for old homes.

Here is a link that might be useful: NPS preservation brief - insulation

There is no reason to remove the plaster walls.
Cavities can be filled with blown in cellulose through a pair of holes.
The cellulose is blown into an ~1 inch hole near the bottom of the cavity, and when it starts to escape at a similar hole at the top of the cavity you know it has filled.
If there are fire stops in the wall you treat the stub bay as separate cavities.

If you haven't pulled off the old plaster and lathe, I would hesitate to blow in moisture sensitive material into a void that could be very damp at times.

Also, consider from the preservation brief linked above:

"A serious problem exists with certain cellulose insulations that use ammonium or aluminum sulfate as a fire retardant, rather than boric acid which causes no problems. The sulfates react with moisture in the air forming sulfuric acid which can cause damage to most metals (including plumbing and wiring), building stones, brick and wood. In one instance, a metal building insulated with cellulose of this type collapsed when the sulfuric acid weakened the structural connections!"

Instead, look to the suggestions in the second post.