Putting a recessed light in over the shower unit
Different countries and even different jurisdictions within a country have different regulations. You should find out what is allowed inside a shower enclosure. One thing that comes to mind is whether this fixture must be GFCI protected (RCD). It could be that in Ireland RCD is present in the master breaker.

But as far as what works, you can just wire the shower enclosure light to the existing one which, I assume, is controlled by a wall switch. In new construction one might have separate switches, but for a retrofit I doubt it would be worth the trouble.