You said Ryanair made that comment though, my point is Ryanair don't actually maintain real European operations from Cork.
The passenger target was set by Neil Pakey, and you'll noticed he "resigned". As you've said only one carrier can deliver that, but not sustainably.
I've sadly no involvement with the route development side of things, but if I was to offer my opinion I'd aim for the following over a number of years.
Lufthansa - Year Round, up to daily.
Transavia - Year RoundParis Orly, up to 5pw
Vueling - Barcelona + Rome, 2/3pw.
SAS - Stockholm up to 4pw year round + Copenhagen/Oslo.
Finnair - Helsinki
Aer Lingus Regional - Bristol and Manchester (when Ryanair leave)
If Aer Lingus get CS100's, it wouldn't be completely implausible for a based CS100 operating AGP/FAO as well as a few other scattered European routes.
One to watch will be Norwegian short haul. If Norwegian choose to expand into the Irish market on Shorthaul, then we will be faced with serious competition from Ryanair. Up to now, it's mostly been Ryanair playing the airports off eachother. Norwegian could potentially be an airline that'll give Ryanair a fun for it's money. Is this sustainable growth if it does happen? Probably not.
Best case scenario situations above, take all of that with a large pinch of salt.