Sure sounds like you may have a couple of unrelated problems. Usually, a nuisance tripping GFI is a sign of a worn or aged GFI device. This is assuming there are no down line devices or lights that are leaking small amounts of current to earth ground. This would be uncommon for a light or simple switch, more likely with a motor load on start up or shut down. The more important and potentially dangerous problem is the whole house lighting flickering when the fan and shower light is on. If you have an accurate digital VOM, you can look at the voltage drop at the breaker that controls the bathroom. If you see variations of much more than 5 to 10 volts then isolate which device is causing the fluctuation. (fan or bath lite) by turning them on individually and test again. A older fan is a common source of your problem as moisture and humidity may have invaded the motor and causing larger than average voltage drop to ground. If this fan is in fact the culprit, the fan may be heating up more than normal as well and could become a hazard. If you do not have a VOM or know how to test voltages inside your circuit breaker panel, I'd advise you to seek a licensed electrician soon to correct the situation.
National Electrical Code is pretty clear on this, at least as of the 2014 version. It says that if the fixture is above the tub or shower, and within 8 ft. vertically from the top of the bathtub rim or shower threshold, the fixture must be rated for damp locations. If the fixture may be subject to shower spray, it has to be rated for wet locations.
If it were me, I'd install a fixture rated for wet locations, just to be on the safe side.
National Electrical Code 2014
Chapter 4 Equipment for General Use
Article 410 Luminaires, Lampholders, and Lamps
II. Luminaire Locations
410.10 Luminaires in Specific Locations.
(D) Bathtub and Shower Areas. No parts of cord-connected
luminaires, chain-, cable-, or cord-suspended luminaires,
lighting track, pendants, or ceiling-suspended
(paddle) fans shall be located within a zone measured
900 mm (3 ft) horizontally and 2.5 m (8 ft) vertically from
the top of the bathtub rim or shower stall threshold. This
zone is all encompassing and includes the space directly
over the tub or shower stall. Luminaires located within the
actual outside dimension of the bathtub or shower to a
height of 2.5 m (8 ft) vertically from the top of the bathtub
rim or shower threshold shall be marked for damp locations,
or marked for wet locations where subject to shower
spray.
NOTE: This code seems to go back as far as the 2008 version of NEC. So as long as your jurisdiction has adopted at least NEC 2008, you should be fine.
You should have no trouble finding fixtures; and/or trim, that is rated for wet locations. In almost all cases, the fixture or trim will be sealed to prevent water intrusion. A quick search through Home Depot's inventory, turned up this item for example.
6 in. White Recessed Lighting Reflector Shower Trim with Frosted Glass Lens
I'm sure you'll find them in all different styles, colors, and from many different manufacturers.
Best Answer
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.