Isn't that the key weakness? If I arrive back in DUB at 3:00pm (say) I can just hit the road, and hopefully be back in Cork in 3 and a half hours. That's a better alternative to waiting four hours to board (say) a 7:00pm departure to ORK.
Give me three hours in the lounge and a flight rather than 3.5 on a coach.
If timed properly there's also a fair bit of domestic traffic that'd use it still - I often enough do DUB-CFN for work, get 6 hours time in DL without overnighting; whereas the KIR timings are useless for that.