Possibly more of a consideration than the motorways is the number of places in Cork and Shannon's potential catchments that have direct public transport services to Dublin Airport.
Waterford, Clonmel, even Fermoy and Mitchelstown have direct services to Dublin Airport, but not Cork Airport. There's no point even mentioning places that Cork Airport would aspire to have in the catchment like Kilkenny and Wexford.
Similarly, at Shannon, places like Nenagh, Roscrea and Athlone have direct services to Dublin.
Faced with a choice of paying for fuel and parking, or taking the bus, for a lot of people it's a no brainer.
I have a suspicion that Cork and Shannon are so fearful of the loss of parking revenue that they don't see the larger opportunity in encouraging bus operators in.
At the moment, Cork isn't even served by buses coming up from West Cork, even though it would only add 5-10 mins to divert. That should be a priority, along with getting some of the buses approaching the city from the North to continue through to the airport. Considering Bus Eireann have complained that Parnell Place bus station has no more capacity, having the ability to terminate somewhere else should appeal to them.