Interesting point. But according to both Aer Lingus and Ryanair, the amount of credit card bookings on aerlingus.com and ryanair.com with Cork addresses for flights out of Dublin are quite small.
Lots of Cork passengers connect via LHR or STN, but rarely from DUB because of Traffic difficulties, expense, and hassle.
I'm not saying that there are no exceptions, obviously, seeing as Cork has a population of half a million people, just that maybe not as many people as you may be implying. You also seem to be implying that an airport near Portlaoise would somehow offer destinations not currently served by Cork or Shannon, but that sounds a bit off the wall frankly.
Two hours to Portlaoise, or two hours to Shannon from Cork, so frankly, what would be the point?
If you want to drive two hours, then why not go to Shannon? It's there already, with real runways, real terminals, and real aeroplanes to real destinations. Why dilute that by setting up yet another airport down the road? This country has about 4.5 million people, yet we already have three state owned international airports and waterford, kerry, knock, galway, sligo and donegal regional airports, not to mention Belfast's two airports two hours drive from Dublin, and Derry airport as well.
The island is coming down with airports. What we need now is a decent road and rail network linking them together.
Not another airport in the taoiseachs constituency so that some property developers can try and make a killing.