Aer Lingus selected 757 as a temporary measure to operate Shannon US routes that otherwise may not be viable . In the interim between when a suitable Airbus alternative becomes available eg 321neo.
In addition using the single aisle jet allows EI to test this arrangement. Also negates the need to add new types to its own fleet. It also enables EI to operate Dublin Toronto at lower cost to Air Canada and make the route viable. 330 much more expensive to operate, and in the lean winter is a great help here. Also enables EI to do routine maintenance in 330 , and 757's slot in, like is the case on the JFK route ex Dublin.