Resurface Fireplace - Apply mortar?
As long as the fireplace draws well you won't have impossible &amp; ugly staining of the marble, so go for it. A rough base is great &amp; will provide the best holding power, an uneven base is bad &amp; needs to be leveled for a uniformly even coat of Polymer Modified White Thin-Set or Mortar...white so the marble doesn't get stained from the backside by your choice of adhesive. This area shouldn't get anything more than warm with a raging fire going.

If you have room for 1/4" or more cement board then great, but be conscious of screw (Tapcon) depth not entering the fire chamber &amp; don't just mortar the cement board on. Screws should only go into the bricks mortar joints &amp; not into the bricks themselves.