According the website, the mirror is meant to be leaning against the wall with the base at least 3.5 inches from the wall. Do you have room on the mantel to set the bottom of the mirror that far from the wall? If so, the part of the mantel that needs to hold 112.5 pounds needs to be the part 3.5 inches from the wall. The cantilevered part of the mantel will hold less weight that the part right up against the wall.

What kind of supports does this mantel have? Is it supported with legs on either side? Or is it cantilevered across the whole span?