We have a 100 year old house with no insulation in an exterior North wall, with limited space (stairs adjacent). The exterior is aluminum siding with no insulation between siding and exterior wall either. Use of plastic to insulate from the interior wall has been suggested, but we were to investigate any moisture concerns before proceeding.
Kindly advise further -- thank you.

Just a thought -- how difficult is it to TEMPORARILY remove the aluminum siding on the outside? You might place an insulating material under it, then put it back up.
If you have unlimited funds, you can replace the siding with new siding that has insulting material attached to its inside.