I agree with both sides of the argument, to a certain extent. I drove from Cork to Dublin this weekend and now that the south ring road runs from ballincollig to the tunnel its a very easy journey. The only enforced stop between ballincollig and Newlands cross is at the dunkettle interchange, also the tolls but they are pretty swift too.
Let's say someone wants to be "in the office for 9". First it depends where the office is, but lets assume city centre.
To get to central dublin for 0900 by car you would probably have to leave most Cork subarbs at roughly 0500. Maybe closer to 0600 if you live to the north of the city and your meeting is at cityWest.
The 0600 train arrives into Heuston at 0830.
To have a chance of getting into Dublin city centre for 0900 you would need to land at DUB around 0730, meaning a 0645ish departure. You would need to be at ORK for 0600.
In short there is no real time saving flying over the train or car, both provide more frequency and therefore flexibility. Of course it all depends where you live and where in Dublin you might want to go. If you live in, say, Ballygarvan and your office is near DUB then the flight would almost certainly be preferable. I think given O&D alone the flights may struggle, but a combination of both markets it may work. As mentioned above though, RE probably have more attractive markets and EI may find flying passengers to LHR/AMS/CDG is a better return than flying them over the atlantic.