I appreciate they may have friends on the Southside, but surely a shorter commute on a daily basis would be better than being close to friends you might see, what, twice a week!? At most...
There are a few fee paying schools on the Northside: For girls - Mount Sackville or even Loreto (in town-easily commutable from Castleknock), for Boys - Castleknock College or Belvedere (again, town-based, but commutable)...or Kings Hospital is only a short trip from Castleknock, and its mixed...