Surly it will be in ad-hoc operation in the next few weeks. It's more less completed apart from last few bits relating to bridge.
It is not unreasonable to provide an amended Cork-Dublin timetable from September without impacting the whole network because it's largely southbound services which will benefit. Additionally there has been minor speed adjustments on sections of Cork/Dublin recently and probably more due long before December.
* 17.00 to Cork loses 5-6 minutes with scheduled stop outside Limerick J and delayed 17.25 ex Cork almost daily.
* 06.15 ex Cork can be up to 8 minutes ahead of schedule passing Portlaoise and still arrive in Dublin late because of the terrible scheduling job done when introduced.
IE's record of a schedule changes over the last few years is add a few minutes here and there usually to ensure OTP remains intact so no reason to wait until December unless they are finally going to provide an updated timetable to reflect useful adjustments since the current 2013 schedule.