I don't think it has anything to do with capacity on the 1305 - the 4 car 2900 provides less seats than the ICR.
The set of the 1305 is used to augment the 1600 to Sligo on Fridays. There are alternatives
1) Run the standard sized 1600 Sligo non stop Drumcondra to Mullingar (or beyond) on a Friday with a 29k relief to Mullingar.
2) Requision an ICR from either the Docklands/M3 Parkway or Grand Canal Dock/Newbridge service and substitute with the 29k currently used on the 1305.