Not a lot of information to go on. I take it you aren't ever going to fire up the fireplace once installed? What are the materials involved? How about pics of other than a viewing position?
"I realize that somebody playing free music isn't as commercial as a hamburger stand. But is it because you can eat a hamburger and hold it in your hand and you can't do that with music? Is it too free to control?" - Don Van Vliet (aka Captain Beefheart) discussing commercial success in the music biz
I would place the speaker on some sort of stand to give it the height you want first, hook it up, and see how it sounds. Being inside of a fireplace, even with the flue closed (I'm assuming it is a real fireplace) you may get unwanted echoing or sound dispersion.