FIRST, you need to ask the manufacture of THIN-SET the type you mix water with if their product works with SLATE. I would think you could take such and trowel into the mortar joints to bring such out to match the brick surface, and then when dry use the THIN-SET and hang the tile. You need to make certain that the first row IS SITTING on the floor or a hard surface, and use spacers to hold the tiles while the thin-set is drying. I just did this with my fire place using 24" square 3/8" thick porcelain tiles. I would NOT use the mastic premix adhesives though.

Keep asking questions and you will find the right answers. I trust I have helped.

Dale
Indy

I was just reading your post from september & was going to suggest covering w/ tile or similar to save the mess & expense of a demo job. Except that it didn't look like you had checked in for a while so I didn't bother.
Heat may soften & loosen mastic. It isn't recommended for use over 140 degrees(that probably varies w/ mfgr.). So you may be safe using it.
Using a cement mortar bed with a thickness of 3/8" to 3/4" is recommended if the temp is above that. The mortar should key into the brick so that lath isn't needed to hold it in place.

We don't intend to use the fireplace at all (frankly, I wish we could just rip out the brick, and cover up the hole with drywall , but I guess there's some resale value in having a fireplace, and we're likely to only be in the house for 5 or so years -- if at the end of 5 years, we decide to refinance and stay longer, than I may decide to go ahead and cover it up). But anyhoo...yeah, we don't use the fireplace, so we just want to do something with it that will look semi-decent for the next few years. We're actually doing the fireplace today, finally...