How can I remove old paint from plaster walls?
The two standard ways to strip paint is heat or chemicals. Both are labor intensive and messy, and chemicals are caustic. I assume you are planning to strip the paint only because you are left with an uneven surface after it was already partly removed? Otherwise, just paint over it right?

It might be easier to lightly scrape to remove loose paint, then skim coat the whole surface with finishing plaster. Some light sanding afterwards, and you're ready for prime coat.