score:7
30 minutes is far more time than you need. Most of the potential delay would be in collecting your ticket: there's plenty of kiosks, but if you run into trouble you might need to wait in line for the ticket counter. After that, it's quite quick to get to the trains.
My guess is that Irish Rail want people to arrive early so they have plenty of time to find their train car from the platform, rather than jumping on the front car at the last minute and then milling about inside to find their seats.