I would use dense packed blown in cellulose insulation. Least amount of damage to existing walls.
- They can drill small holes either in the plaster or the siding and fill the wall cavity.
- When done properly, the cellulose will also limit air infiltration.
One of the biggest problems with old houses is there is no air barrier; they are very leaky since the sheathing is 1x6 boards rather than sheet goods like we typically see today. Cellulose technically does not qualify as an air barrier but performs very well.
Cellulose is also a very "green" and safe product. Made of recycled newspaper and is all natural so very safe for the occupants of the home.
Be sure to use a qualified experienced contractor because if this process is not done properly you can end up with lots of settled and missed areas.
It is helpful to have the work inspected with an IR camera to be sure there are not any voids due to purlins, etc.
For more information:
Read "Doesn't sprayed cellulose insulation tend to settle, leaving areas in the wall where there is no insulation?" a Q&A answered by Lily Livingston.
Also, read "We have inadequate wall insulation. What is the best approach to insulate our walls?" a Q&A answered by Michael Holcomb.