All the carriers flying between the USA and Shannon operate the 757, an aircraft perfect for Cork's short 7,000ft runway.
The DAA should encourage Aer Lingus to relocate their 757 US operations from Shannon to Cork or at least one of the US carriers, United, Delta or American to relocate their US services to Cork where they could operate a year round 757 services instead of just seasonal services to Shannon and with zero competition too!
No doubt they have, but airlines go where the demand is, and Shannon has an established TATL market.