A 777 did take off from a runway in Dumplin heading to the US, that runway is 500 meters shorter than the main Cork one.The problem with that is transatlantic flights from Cork would be in direct competition with already established flights from Shannon. Shannon has pre-clearance. And while Shannon has lower passenger numbers than Cork it has one ace up its sleeve. It has the longest runway in the Country. 777s and A330s cannot take off or land in Cork.
Something technical about the design of the wings i believe