Do not use plywood, full stop. It will eventually expand/contract, and you're tiles will move with it. Plywood is a natural material, and behaves as such, and this is something you don't want, behind tiles.
Use something like Tegral Hydropanel board, Versapanel, or Hardibacker 500. These are NOT wood-based boards, are impervious to water, and are temperature stable.
I used Hardibacker 500 in 6mm in my bathrooms - including walls - sheets are small too, so easier to handle. Insulation Distributors in Dublin are the importers iirc, but I think Chadwick's stock it.