Correction there - The Cork doesn't have a guard. The driver looks after the doors with being waved off at the manned stations while the Train host looks after the passengers and the checker checks their tickets.
There's no checker on Cork-Dublin trains any more, with the exception of early and late services when the booking offices aren't all open en route. There's a couple of RPIs who show up from time to time, generally between Limerick Junction and Mallow or Mallow and Cork, and sometimes a guy who sits in first while the train is on the platform in Dublin and sells excesses.
The train host is fully safety trained and can do all the things a guard used to do when needed. They handle wheelchair ramps and manual announcements when needed. Beyond that in general around half of the train hosts sit in first and read a magazine and the other half go up and down the train looking to help people, go to the buffet car for elderly or limited-mobility passengers, and what-not.
Each station has a dispatcher who uses a white light or hand signal for CD and a green flag for RA.
Well I doubt the Train Host is paid anything like a Guard's wage, probably the same for the ticket checker.
Many train hosts used to be guards and would be protected on their old wage, and given their emergency duties I'd be surprised if the unions had accepted much of a decrease.
The Enterprise also often has no ticket examiner, but one isn't especially needed as all stations on the route bar Dundalk are gated. The guard behaves similarly to Virgin TMs and sometimes locks off a local door etc.
The 22K services sometimes have a ticket examiner and might have a catering trolley, but are properly DOO.
The Irish Rail Mk3s would need to be rebogeyed etc. to work in GB given the gauge difference (1.6m vs 1.435m).