I also have an old house with the plaster horsehair walls. I can't believe what some peoole do to these old houses well anyway try this...fill up a spagetti pot full of hot water then pour in a small bottle of white vinegar about I think it is 20 oz the small bottle. It works miracles on old wallpaper but I don't know about the paint over the wallpaper. But maybe if you let it soak enough it will chew through the paint and down into the paper and you will be able to scrap it off.