What is the fireplace made of? Brick, stone, tile, marble? Tile needs a flat smooth surface.
Here is one guy's makeover over brick.... https://www.youtube.com/watch?v=69UX9mGsv1A
If you aren't going to put cement board to lay the tile, you can try powergrab. We used that on a tub surround.
I put cement board around the opening and then used thin-set to install tiles. However, I'm a big fan of Power Grab since then. I use it to install back splash tiles in the Habitat for Humanity homes. It comes in a caulking tube and you just add a bead to the tile and press in place - NO MESS. You can also get grout in a caulking tube. Less mess and minimal clean up.