Within 1km of where I'm sitting in Cork Airport, there are half a dozen US companies (Amazon, Marriott, Bank of New York, Avery Dennison, IBM, Red Hat).
That list doesn't include the major US employers around the Cork area (Apple, EMC, Boston Scientific, VMWare, Pfizer) and a whole host of smaller US presence. Yet there's never a sniff of a US route from Cork.
It's no surprise really. So many employees end up on flights from Shannon and the rest connect via Heathrow and Amsterdam, so there's no benefit for Aer Lingus. Any other airline needs to look at how much profit they can actually make and whether Aer Lingus would respond if they launched a US route.
The same applies to a route to Canada Northern Ireland. Airlines will take a cold look at potential profit and how that balances with the risks involved and shy away from it.