Wondering whether the weight of the mirror may be too much for the mantel. EA from Tenafly, NJ Dear EA, It depends on who installed it and how.
Plaster is a difficult material to hang stuff on.
I have found plaster walls to vary dramatically in their ability to hold picture hangers. Of course, the strongest method is to install the screws into the wall studs, though the studs are seldom in the right place. A subcategory of Murphy's Law, I think. Start the screw hole with a drill bit smaller than the screw. You can use a masonry bit, but I have found that regular bits will work in most plaster walls but will become dull.