Interesting to see that IE's annual passenger figures dropped massively by getting on for 20% between 2007 and 2012 following the global recession, whilst Britain's figures grew by roughly the same percentage during the same period.
Pleasing to hear they are bouncing back now.
Good news
I'm no expert, but I believe that until the economic boom, there was little rail commuting or business travel in Eire outside Dublin, and so part of IE's response to the loss of subsidies and declining ridership was to cull services, which didn't help passenger numbers.
Even now, long distance service density is pretty low, and services no faster than the motorway, so unless you are going city centre to city centre, it's probably faster (and more convenient to take the car). For example, Cork to Dublin (which is a bit further than Cardiff to London) is 2h15 at best, and mostly 2h40, with a roughly hourly service, although it's a snip compared to
GWR▸ at €70 return. Wexford and Waterford (roughly the same distance as Bristol to London) to Dublin are about the same timings, so not really commutable, even though they are even cheaper.