Based on Tigger1962's advice (big thanks!) I've decided to try both three & O2. They both cost approx 20 euro a month & have a 10Gb download limit. I think Vodafone is around 30 euro a month with a lower download limit - and seemingly is not so good between Dundalk & Dublin.
Last Friday evening I tried the three modem on the 18:40 Dublin Connolly to Dundalk commuter train & was disappointed with its performance. This morning I tried the O2 modem on the 07:57 / 08:00 (depending on whether you believe the Irish Rail or Translink timetable
) Dundalk to Dublin Connolly Entreprise train & found it quite satisfactory.
I wasn't really taking note of exactly where the coverage was good / bad, but just forming an impression of the overall level of coverage during the journey. I'll have to make up my mind this week which is better as O2 only give a 7-day money-back guarantee - with three & Vodafone it's 14 days.