If it is a really big, heavy mirror with a wooden frame, you can use these:
Mirror Plates.
Fix them with two screws in each plate to the back of the mirror first (two at the top and two at the bottom, or on the sides). You should drill pilot holes and use short screws. Get some help to hold the mirror in the right position and mark the single hole positions with a pencil. Take the mirror away and turn the pencil dots into bigger "+" marks (so thet you can still see where the hole should be after you've started drilling).
Start by drilling slowly (if you can) without hammer action. Once you're through the plaster and into the brick, you can speed up and use hammer action. Keep the drill bit absolutely perpendicular (at 90 degrees) the the wall and drill to the depth of the flutes on the bit. (If your helper hold the vacuum cleaner nozzle a couple of inches under the hole, there'll be a lot less mess to clean up afterwards.)
Push your first plug in and get your helper back to hold the mirror up. Put the first screw in most of the way, check that the mirror is level and check your pencil marks for the other three holes. If you made a good job of the first one, everything should line up. If not, re-mark the holes, take the mirror down and try harder not to let the second hole wander!
Once you've drilled all the holes and put plugs in, it's time for your helper to hold the mirror again while you put the top two screws in most of the way. Now, while your helper puts the kettle on, you can put the bottom screws in and tighten them all up.
For a really professional finish, use brass slotted screws and line all the slots up so that they are vertical. That should earn you a biscuit too!!
