S M Wynne has given you the most sensible answer, and is the correct method.
If you take the advice of RJS and Substructure, ie pva the walls prior to tiling, you will have nothing but trouble.

PVA Reactivates to water/moisture.

Yes you can tile straight onto the cement wall, the rough surface will actually act as a better key for the tile adhesive than a smooth skimmed wall.
You might just want to check that the existing isnt loose or damp ect...
Hope this is of some help.

I would advise you to remove any loose plaster.Then to pva the surface and rather then skimming the whole wall with new plaster.Apply bonding to the damaged arears troweling flat.
Bonding is a backing coat for plaster,then pva this surface and tile straight on top.
Hope this helps.

rjs is right ,bond the walls flat ,this will save a fortune in tile adhesive ,and give a better finish to tiles

rjs is correct make sure there isn't any loose plaster,bond over the gaps on the walls to bring them as level to the assisting wall make sure its flat and smooth for when u apply the tiles,rather then paying for materials and labour to re skim the whole bathroom. hope this was some help for yourself cheers