Tile over Plaster (bathroom)
You may not have lathe, but compressed board under the plaster and scratch coat. Either way; if plaster is any way compromised you should tear out to studs and start over. Bring plumbing to code; add a diverter and shower line in the wall; cover with cement board, tape off. Follow manufacturer instructions to a tee. Or, flatten walls with a latex modified skim coat product. When dry, waterproof with Custom Building Products Red Guard or better, Schluter Kerdi. Do not use type 2 mastics with these products. Please follow instructions properly. Showers see more water than a rain forest. Showers are labor intensive. Do it right the first time. Hope this helps.