I can see FR172/173 brought earlier around 7AM/8AM like the existing Aer Lingus DUB. Then an additional flight around noon-2PM. Then keep the current FR174/175 as is.
This would mean that RYR would have to have interlining agreements with EI or is there other airlines such as DY, UA, DL where RYR could have US interlining at DUB.
US Pre-Clearance was a good facility, where PAX going out to USA would clear US customs on arrival to DUB.