Reservations are available, but not required, on Enterprise services.
It's a little bit more complicated than that, depending on the class and direction of travel. Quite frankly, it's a mess.
For Enterprise journeys starting in Northern Ireland, first class reservations are available from Translink by telephoning 02890 666 630. Standard class is unreservable, except where the journey is the return leg of a ROI-NI-ROI journey - then a reservation can be made at the same time as the reservation for the outward leg.
For Enterprise journeys starting in the Republic of Ireland, reservations can be made at
www.irishrail.ie using the "Free Travel Pass or Valid Ticket" option. Standard class reservations are free of charge, but it is not possible to make a first class reservation without purchasing a €10.00 first class upgrade at the same time. There is no official way to just reserve a first class seat if you already hold a first class ticket (although this
might be possible if you telephone Translink).
All other trains in Northern Ireland are standard class only and entirely unreservable, as are Commuter trains in the Republic of Ireland.
Intercity trains in the Republic of Ireland are reservable in the same manner as Enterprise journeys starting in the Republic - all trains are standard class only, except some Dublin-Cork/Tralee services (on which it is also impossible to just reserve a first class seat without buying a first class upgrade too). Note that it is not really worth paying for the upgrade on Tralee services or those Cork services where the upgrade is priced at €6.25, as all you get for your money is a table lamp, it is still 2x2 seating. Only those Cork services where the upgrade is priced at €12.50 are
real first class.
