I take the point about the motorway but if the same logic applies in reverse, why not insist that all Uk and European flights leave from Cork. It would provide critical mass for airlines and allow them to open new routes. I'm saying that with Tongue in cheek. Logically Cork should be able to support at least one transatlantic service if Shannon in the open skies age can support 4.
The runway is not a barrier although I accept that the Cork brand name is probably not known in the US. Maybe calling it Blarney Stone Airport might help!