Clearing US immigration at DUB is only available for certain flights just now. However in 2010 with the opening of the new Terminal 2, not alone will clearance be available for all flights to the USofA, but one will be able to clear US Customs and Agriculture as well. Your onward flight to the US will now be treated as a 'domestic' flight on arrival. This will afford all airlines using DUB to fly into 'cheaper' airports in the US. Perhaps this is what Ryanair is looking at just now?