I assume from the initial post that the objective is to insulate in the most non-intrusive manner. To Insulate the walls, there will need to be a hole into the cavity somewhere, inside or out side doesn't matter ( If the building was built with balloon framing, you may be able to access most of the gable end walls from the attic.)
If possible, I usually try to do this from the outside. You don't need to remove all the siding, just enough to access each cavity. If you have horizotal lap siding, just one row of siding will be enough in most situations to access all but above the windows (If ther is vertical panelling inside, more may be needed because of the blocking need to fasten the panelling).
If outside isn't possible, you can put holes on the inside of the walls. I've done it where a row of holes at "Chair rail" were drilled & covered with a... well... Chair rail moulding.
As far as materials, the 2 best posibilities are Icenyne foam or cellulose. Don't worry about a vapor barrier as both of these products are air leakage barriers (air leakage is the primary mechanism for problem moisture getting to where it shouldn't be.) If you are concerned about the vapor barrier, you can always paint one on.