Steaming and Scoring
Removing paper from horsehair plaster is a delicate business. Because the plaster is incredibly fragile, it requires a light touch. Hence, steaming is the preferred method of choice. First, score the paper with a wallpaper scoring tool; this allows the moisture from the steamer to penetrate and attack stubborn old glue. While scoring the paper, turn on your steamer so that it can reach the appropriate temperature and be ready for use.
Steaming and Peeling
After you score the wallpaper in the entire room that you wish to remove, apply your preheated steamer to the wall, working slowly and patiently to loosen the adhesive. The sheets should peel right off of the wall once the adhesive releases its hold.
Complete each section before moving to the next. As old glue is stubborn, it will likely be necessary to steam the section more than once to fully break the adhesive bond.
Alternative to Steaming
Steaming is the absolute best way to remove wallpaper from horsehair plaster (due to the fragility of the wall). But if your paper has been painted over, steaming will likely not penetrate to the adhesive layer. You will need to resort to old-fashioned scraping.
Using a wallpaper scraper and a light hand, scrape the paper from the corners to the center of the room. It is essential that you do not apply too much pressure to the wall, as you will most certainly damage the plaster.