I’ve spent a frustrating couple of hours researching how best to adhere tile on plaster walls that are in decent shape…or will be shortly, after I’ve scraped off the loose bits of adhesive that was holding the hardboard in place, to which was attached some nasty plastic tile in several spots, over which was glued Kelly-green beadboard. (Then maybe do a few brown-coat patches.)
I have been in countless old homes that were built pre-drywall/cement board/Kerdi board that not only have original plaster walls, but original tile on said plaster walls. Hell – there are mosaics set in plaster that have survived for 2,000+ years. Under water.
The internet is full of opinions – but even those from recognized experts are contradictory.
“No! You MUST secure Kerdi board/drywall/cement board to the studs and tile to that!” Um, no. There are no studs. It’s solid masonry.
“Yes! Just paint it first with RedGard.”
“Yes – but whatever you do, don’t paint it with RedGard!”
“No! You must remove all the old plaster down to the brick, then install lathe and hang drywall to that.” No effin way.
“Yes – but don’t use/only use thinset.”
“Yes – but don’t use/only use mastic.”
“Yes, but only if you use mastic you’ve rendered yourself from Pistacia lentiscus.”
“You fool. Don’t you know the correct term is Ceramic tile adhesive?!”
“No. You should just move.”
Rude Mechanicals Press Blog 
Have you looked the national park service old house preservation guide. This details how houses are to be repaired that are under their control. I have had good luck with it.
https://www.nps.gov/tps/standards/four-treatments.htm
I have – I found info there on restoring plaster, and on restoring/caring for tile…but nothing about applying tile on plaster 😦
Personally (as a Euro, still building with tile-on-plaster), this assh*** seals the plaster with something (any ole paint works) to stop it sucking all the water out of the adhesive, then uses tile adhesive, spread with a comb trowel. I’ve never applied tile with mastic, and have heard of adhesion issues down the line in damp environments.
Thanks! I wasn’t smart enough to think of asking any Europeans, and should have – lots of masonry buildings there.
The last one made me laugh out loud!
Have you given Ron Herman a call?