The Irish Rail website has an FAQ which says:
Intercity passengers should arrive at the station 30 minutes prior to departure. If you are collecting a reservation or purchasing a ticket especially during our peak travel times please give yourself more time. Gates will close 2 minutes before the train departs.
As a user of trains in the UK I found this very surprising -- turning up half an hour before the train is due to depart in the UK would be entirely unnecessary, even if you needed to buy a ticket.
What is the situation in practice on the ground? Are there always 15 minute queues for ticket machines, or some other reason why Irish Rail recommend this early arrival? Would I be denied boarding (seems unlikely given the "gates close at 2 minutes" note)?
(For context, I'm arriving in Dublin on a ferry with a scheduled arrival time of 13:45, and wondering how likely it is that I will be able to catch a train leaving Dublin Heuston station at 15:00. At the moment it seems like that should be doable assuming an on-time ferry and a taxi to the station, but if I must arrive at the station 30 minutes early it looks a bit more dicey. My fallback plan would be to get the next train, but that is not until 17:05.)
Update: I also found that the Iarnród Éireann Online Tickets Terms and Conditions include the line
***Please take your seat 20 minutes in advance of departure***(though whether a "please" request is actually a legally mandated condition I'm uncertain of), which would line up with Sneftel's theory that they want people to board early rather than predicting long ticket machine queues or similar in-station delay.