The problem is Northern Irish Railways who refuse to play ball to assist in developing the service. I've had the strange experience of going to two north south meetings on the Enterprise service, Irish Rail and Translink attended in force for Belfast, Translink did not send anyone to the Dublin meeting.
Translink's journey planner does not support rail timetables south of Dublin, nor does Translink provide its timetable to any third parties, e.g.
www.bahn.ie has the full Irish Rail, UK, French, German etc timetables loaded but nothing for Northern Ireland.
My understanding is Irish Rail have offered Translink a very straightforward deal which would bring Translink into the 21st century in terms of online sales and proper booking. There will be fit out of Wifi in the future, guess who is holding that up?
The evidence is clearly one of Translink holding onto the status quo and actually walking away from offers to solve the issue.
A dedicated visting Ireland page will be provided on the new Irish Rail site in early 2012. It was one of the RUI recommendations and Irish Rail has agreed to provide after the new site has fully replaced the old one.
Incidentally our colleagues in the European Passenger Federation are amazed at the fact full through ticketing applies across an international border its quite unique in Europe.