All US flights except 1 EI to JFK and 1 EI to BOS use USPC.
How do they decide who does and who doesn't clear in DUB?
There is only a certain number of gates which can use USPC and there can be more US flight departures than gates which is why USPC is not available for all flights. I think it's 5 gates.
The reason Aer Lingus don't use it for the above flights is the time of day for second JFK and second BOS only goes 3 days a week.