Paul, probably the Irish HSA doesn't want people using BS546 plugs in theatres because some of them are quite fragile.
Just like the domestic (cheap) BS-1363 plugs, some BS-546 are made out of the brittle thermoset plastics.
Of course, I've seen heavy duty rubber BS-1363 plugs from MK and others...but they're expensive -- like two or three quid versus a few pence for a no-name "domestic" plug to a pound and change for an MK type.
