I have unpainted brick around a wood fireplace. I've purchased the brick, Eldorado Stone and they said to use Type S Mortar.
I was told since the brick is unpainted that I can directly apply the bricks with mortar to the existing fireplace.
I have also heard from others that it is better to first put on a scratch coat and then apply the new brick.
Also told to attach "chicken wire" so the motar will adhere better.