You need to test how well the paint is stuck to the wall. To do this stick a piece of duct/gaffer tape to the wall in several places, press down well and leave for 10mins. Then pull the tape off, if the paint comes off with it you need to remove all the paint by scraping (not stripper) If it stays put you can tile straight onto it with a decent premixed or powdered adhesive (not adhesive & grout)

You can again go straight onto the bare plaster if it is sound, if a bit dusty and friable then prime with a suitable primer NOT PVA. Your adhesive should give a suitable primer, I tend to use BAL adhesives and would give it a coat of diluted APD.

Jason