Called wall anchors or wall mollies these are small tapered plastic anchors that you push through the plaster after drilling a correct sized hole.
How to hang heavy mirror on old plaster wall.
Hang a mirror using hooks step 1 hold the mirror in place close to eye level.
Interlocking brackets on the wall and the mirror provide secure and level hanging.
Expand the anchor.
Mark the wall near the center of the top edge with a pencil or painter s tape.
To hang mirror on a plaster wall use a toggle bolt outfitted with a picture hanging hook.
Drill hole through plaster large enough to accept the toggle bolt.
Thread machine screw through toggle bolt nut then pinch flat the wings on the toggle and push it through the hole in the wall.
Hanging heavy mirror on plaster walls.
Center the mounting bracket or cleat on the back of the mirror frame and level with its top edge.
A variety of drywall anchors are available.
Use a power drill to carefully make pilot holes into the frame using the pre drilled holes in the bracket as a guide.
Your mirror is too heavy for picture hooks alone but a wall anchor would likely do the trick to attach them to a wall made of drywall or plaster.
One foolproof method to hang heavy mirrors or picture frames is to use a molly bolt.