[legion]

Some companies will do media blasting with powdered dry ice. You probably don't want to spend the money for creating such a setup yourself.

Another alternative is using a heat gun. This will work especially well if the paint is not adhering well anymore.

Even if the paint is peeling in sheets in some places, it is still probably well adhered in other placed, so a combination of methods will probably be your best bet.

Any method that is used will still leave paint chips all over the place. And don't be surprised to find out that the paint was put up to hide decaying mortar and you need tuck pointing when it is all done...