Askeroid - Those pre-clearance checks for US customs and immigration are as you say very advantageous to any operator flying out of DUB or SNN. Opens up all the secondary airports in the US given that pax can simply arrive as if on a domestic flight. Ireland is the only country in Europe with this facility. Of course, it also saves the airlines from bearing the cost of shipping back pax who are denied entry into the US once they land.
SNN and DUB would serve as ideal feeder points for a FR run service across the Atlantic. Of course, the ugly sister in all this will be ORK.