The FO problem on the 1815 arises because of the significantly reduced evening capacity ex Dublin to the Mayo line in recent years. Traditionally there were two trains on Friday evenings, one for Westport and one for Ballina. Now there is only one train. While the 1445 service is very welcome, it is not an evening service and does not relieve the 1815 to any great extent - hence the requirement for the 1710 being extended to Westport on Fridays.
As it stands the 1815 from Westport and 1710 from Heuston at at either end of the same single line section - either you delay one or the other for at least 20 minutes. If you bring the 1710 forward to Castlerea, there is oodles of time for the 1815 to serve the longer platform and then shunt to the loop to await the arrival of the 1710. Not sure that the case for delaying the 1815 is compelling unless the schedule change is properly advertised and loaded into the journey planner.
As regards extending the 2nd platform at Castlerea, there is no impediment other than money in extending the platform - the level crossing is not an obstacle. Whether it is a good investment for one train a week is debateable !