Nail a cleat to the floor and lean it against the wall.

One suggestion would be to hang it on two heavy-duty hangers, distributing the weight. I was told to first put a piece of cloth medical tape on the wall where I was going to nail in the hanger (I have the same walls) so if you need to remove the hanger it doesn't pull the wall out with it. Another alternative (which might be better anyway) would be to call a cabinet maker who has experience hanging cabinets on plaster walls. They might have some good advice.

Heavy duty picture wire and one of these little doohickies would do the trick, actually. I use them to hang everything, including an old mirror that weighs close to 40lbs. It's been up in a high traffic area since I got it over a year ago and the drywall still looks pristine there. I'm sure if you were able to find the space between two laths I know the hook would do well in the wall.

I'm with meredith. Call a carpenter and see what they can suggest. I love your mirror and think this would be worth the expense.

Hang it from a stud. Put tape over where you want to drill a hole to keep the plaster from cracking and make sure to pre-drill the hole.

Regardless of what your walls are made of you should still have studs. Find one and put your mounting hardware there.

I have the same walls. Find a stud. And get the right, heavy-duty hardware to ensure your hangers don't pull out of the back of the mirror frame.

Beautiful mirror! If the mirror is wider than the stud spacing (typically 16" apart, but it can vary so it would be good to check), I would use very strong screws to string a wire across the back of the mirror (being very careful not to drill too deep and damage the front of the mirror!). Then I'd find 2 studs in the wall and put 2 long screws in the studs. You can then hang the mirror by the wire on both screws, and distribute the weight across 2 screws. But I guess I'm a little paranoid about things falling down!

Other options: If the mirror is big enough to anchor between two studs, you can use a 'french cleat', or you could hang it from the picture molding which is often found in plaster-walled homes (if you have that).

If you are interested in using an easel still I would talk to an art supply store because I know they could sell you one that would work with the weight of the mirror. I have seen some pretty large pictures being displayed on easels.

you can hang anything on plaster that you can hang on drywall, like others said above, you just need to hit a stud. You can buy $20 electronic stud finder shipped off amazon.com, first new tool I bought after moving into my house which is a 1913 home with very less than perfect plaster walls. I have a heavy 62"x50" baroque frame with 7" frame width hanging on my wall. It's hunge from two hooks on two studs.

You don't need to find a stud -
Just use a pair of 50# picture hangers from the local hardware store:

http://www.shop.com/51_7185_50_BB_PICTURE_HANGER-110381811-137580565-p+.xhtml?sourceid=1414

Even better - if your apartment has picture rails, use a pair of these for hanging:

http://swanpicturehangers.com/rail_hooks.shtml

hang from the ceiling or get a smaller easel and a nice console table.

I have 2 big square mirrors leaning against the wall be hide my sofa because their 2 big and heavy to hang on the wall.

Thanks everyone! I will call a carpenter to start to see what they suggest and see if they think it is possible to hang it on the wall (which would be ideal!) I'll keep everyone posted!

I really appreciate everyone's suggestions!

You could totally lean it against the wall on a sofa table or accent table of some sort. I saw once that someone used artwork in a foyer and they displayed it by leaning it against the wall and resting it on a very long and narrow sofa table. They also tied it together with matching lamps on each end. My best friend always complains about having thin walls that she can't hang things on...it sucks but it's an interesting challenge. Goodluck! :)