How can the aircraft gain an hour?
Actual flight time from runway to runway is minimal on the Irish sea hop. The distance between DUB and LHR is around 450 miles. An 737/A320 with a top speed of 550MPH is able to fly that in less than an hour (ATC/climb permitting) yet most airlines timetable for a trip time of 1hr20 to 2hrs. Presumably this is to allow a little slack for delays into congested airspace etc... Therefore it is quite possible for a flight to make up time if it doesnt encounter the problems and delays that usually affect the route.