Claimable tax expenses on rental property

It’s all about the facts of a particular case really.
Absolutely.
I want to replace the bath with a shower unit, tank the walls to prevent water running into the sitting room. Add warm board to the external wall to prevent damp. Put in an extractor fan. Replace wall and floor tiles.
I just don't see how installing items that were not there previously (shower, tanking, warm board, extractor fan) could possibly constitute "repairs". The OP isn't simply replacing items that have worn out or been damaged with similar or comparable items.

It seems to me that the installation of these items are enhancements to the property and would be treated as such by Revenue.
 

While on a strict view that's certainly correct, I think you need to bear in mind that the only point in time that this is likely to be reviewed, is after the fact. The tax inspector has no realistic means of knowing the specifics of what was there beforehand...

So, depending on what paperwork is available to illustrate the type of work that was involved, it's likely that one would accept an invoice describing the works as repairs to remediate leaks and install a new shower unit - it's entirely plausible that the works were in the nature of repairs. Obviously, that's a bit of a fudge from the taxpayer's perspective, but I doubt it would cause too many people to lose sleep over it in the grand scheme of things!