I'd believe that if it were true but from my experience it's not. On a number of occasions when I've been on the 11:00 Dublin-Tralee, people have actually been left standing from Mallow to Killarney and that train is always €10, except Fridays.
The soon to be axed 20:00 Dublin-Tralee on the other hand is a very quiet service, yet it's always €20. The 19:15 Tralee-Dublin is similarly quiet and I've seen no recent increase in numbers that would justify a sudden doubling of the fare. And, as I've already said, that train connects with the 20:30 Cork-Dublin at Mallow and that remains €10 from Cork (as does the connecting train from Limerick) so this can only be interpreted as blatant discrimination against those using this particular service from Kerry.