I think that once Ireland aligned themselves with the EU, they received SO much inward investment that upgraded infrastructure and facilities all ovewr the country, though especially on the East coast/Dublin area where all the political decisions are made, it would have been unlikely that they wouldn't be a client state for the EU aircraft corporation ;-) despite many many years of loyal homage to Boeing, no doubt in part due to the ethnic Irish Americans.
Both the McDonnell and Lockheed products would have worked on the then route map, especially as the airline was handcuffed to the Shannon stopover for most of the US flights. Perhaps the smaller capacity of those aircraft might have been more profitable for Aer Lingus, especially over the leaner winter months.