I'm not on oceanic controller, however I do work on the sectors that join the oceanic airspace.For departures from EINN,EIDW and scottish airfields which enter between 55N10w and 58N10w we will pass an updated estimate to shanwick and unless they tell us otherwise we will assume that the clearance previously given stands with the new estimates. Traffic going north of 58N will pass their time revisions themselves either Via the CDO frequencies or you can tell us and we can pass it along.
I'm not sure whether shannon works to the same rules but would have thought that it was not that much different to ourselves