I don’t think there’s any great stream of Cork people up to Shannon any more. Because of the motorway link and very regular bus services, Dublin seems to be the second choice now. The same applies in reverse. Only really in North Cork and West Limerick are the two airports really competing these days. That’s only a combined population of 150k.
For early flights it's still a haul to get to Dublin in time - even more so if you have to deal with the American formalities. The tunnel has made a huge difference in accessing Shannon but the Buttevant-Charleville situation needs sorting. Unfortunately the powers that be are insisting it be a motorway rather than decent relief roads. I came down from Shannon a few weeks ago - after 15 years in Canada the N20 south of Croom is largely unchanged. We were stuck in Charleville traffic for ages because there was an event in the town square. Ludicrous.