My guess is that either that:
a) There was a planned platform opposite the current platform 10 for the proposed Phoenix Park Tunnel services (stopping at Heuston);
b) or when the 'far' platforms (6-8) were built, there was a planned platform 9 opposite platform 8 as another platform. There seems to be ample room as there are already a few tracks there with the possibility of another platform being put in.
Another (slightly off-topic) note is that next to platform 10, there is a concrete structure with a shelter/cover and signs displaying 'Ardán/Platform 1'. Not sure what this was ever built for, it looks like the shelters over platforms 6-8.