Think the route most impacted by a US route ex Cork would be Heathrow rather than Shannon or Dublin airport US services.
For the unlucky passengers from Stansted and Manchester who were diverted to Shannon the other night due to strong south westerly winds, I'm sure most would have preferred to be diverted to Dublin where at least there is a proper motorway to Cork rather than having to endure the Limerick to Cork boreen on the back of a bus.