If you didn't have a window there, I'd suggest painting the wall in an accent color from the painting, but I like accent walls to be solid, making an exception for FPs. Paint your brass on the FP with heat proof matte black paint. Try hanging the mirror horizontally as it is filling up too much vertical space. You need to have several inches open above the mantle and more space between the mirror and ceiling.

If possible, I would hang it on its side, since there is little vertical space. Then I'd get a big black round vase with a silk orchid to put on one side of the mantle.

First of all, I'm afraid that your mirror seems too big for the area where you've put it. Then your art work and your mirror are 2 really big different statements. I think they would look better in 2 different rooms. In this room I'd put white moldings on top of the walls in order to match your fireplace.

I agree with Margo- I'd get rid of the brass. I'd also hang the mirror horizontally.

Are there any tables or lamps in the room? Everything will look better with good lighting...and yes to everything above.