Most airports with a curfew have a fine for late carriers rather than a requirement to divert. The fine covers things like extended control cover, plus any noise pollution breaches.
If the curfew was set for midnight, no airline would want to operate a service that was scheduled to arrive after 11pm (arguable EIR would need to go with 10pm), if it meant a diversion. Delays happen and as often as not one of the aircraft arriving back into Cork at night has a delay of around an hour on the flight.
Looking at today's flights, we already see 45 mins on the aircraft operating the BRS flight.
Ryanair operate to/from Paris BVA up to 23.15 and the curfew is from 23.30, they have very few problems and always prioritise where possible to meet the 23.30 curfew. Either EI or FR seem to have a problem at NOC and they manage to stay open to cater for delayed flights.
00.00 in winter is acceptable but possible 01.30 in summer and possibly a 90 minute delay period once they get some notice which most of the time airlines will know if there will be a significant delay to operations.
I assume the airport doesn't need to be operational with no ATC so it could save a lot of costs.