If you don’t want to touch the inside how about putting vinyl siding, they automattically put tyvex (insulator) on the outside walls. My friend did this and the walls inside feel nice and cozy. It definitely mad a difference!

It is impossible to get insulation behind a plaster wall. You would have to get a insulation board and then apply drywall over that. If you do tear down the plaster walls have fun it is messy!! you may want to check out a hotel at that point.