Designing Kitchens and Bathrooms - tile on plaster?

My bathroom walls are lathe and plaster, and I want to tile them, AND build a shower.
Can I just tile right over the plaster on the dry walls?
What about on the shower walls? do I need to use backer? membrane? poly? some other waterproofer?
the plaster seems to be very well adhered to the lathe
Thanks!

If it is real plaster, it is too smooth for most adhesives to stick, long term. You will need to rough it up and/or cut grooves for thinset to stick. Better would be to remove the old plaster down to the studs and install duroc or hardibacker. Adding a backer over the plaster cuts down interior space. A 1/4 or 1/2" doesn't sound like much, but you would be surprised. If you tear it all out, do install a moisture barrier on the studs, otherwise, no.

stripping all that lathe and plaster just doesn't excite me, and the room is plenty large. Can I get by with just the 1/4" backer on the walls? If I do it that way, should I put a barrier between the plaster wall and the new backer?

The rest of the room can have the treatment you suggest, but I would strip the shower area to the studs, install a barrier, 1/2"(min) backer board, then tile. !/2+1/4, glued togehter, would be better.

when i put the 1/4 backer on the old plaster walls, should I use thnset or mastic along with the screws? v-trowel or 1/4 notch?

On plaster, I would use some other adhesive. Not sure thinset will work as well on plaster. Roughing up the plaster will allow things to stick better. Other comments are welcomed, since I am not an expert in this area, but I have done quite a bit of it.

thanks for your help!