For completeness, getting off the ferry in Holyhead meant waiting with our bikes and foot passengers for a bus to drive onto the ferry, which then shuttled us to a building where our passports were checked. Basically last off the vessel before walking through to the train. If the same sort of thing happened at Dublin Ferryport, there would be no chance to make a 45 minute connection to Dublin Heuston, even at Tour de France pace.
That said, Dublin let us ride up to the ship's ramp (walking the steel ramp and deck), no messing about with bus transfers from a building. Perhaps they'd let us just ride away from the ferry in Dublin, which would save some time.