How to Hang a Heavy Mirror on Different Types of Walls
Although it's preferable to hang a heavy object from a stud in the wall, studs aren't always exactly where you want them. Instead, use wall anchors to secure a heavy mirror. The weight of the mirror and the type of wall it hangs on are the key factors in selecting the right wall anchor. It's good practice to choose a fastener that can hold more than the required weight. Most fastener packaging will tell you how much weight the product can hold, what type of wall it should be used on, and how to install it (including the drill bit size, if necessary).
D-rings, keyholes, and wire-mounted mirrors can all be hung on an anchored screw. Just remember to leave a quarter- or half-inch sticking out. Brackets, on the other hand, need to be in place before screws can be installed.
http://wellhung.net.au/