Up until the early 1990's there basically was a skeleton service on Sundays until the late afternoon, no suburban services at all. It was fairly easy to find a few trains spare to run extra services, even though at times this led to cancellations. Now trains are busy 7 days a week and there is limited scope.
There were massive complaints of overcrowding back then, anti social behavior, trains damaged. Irish Rail are not keen on Dublin football away games after getting a train completely trashed in the mid 1990's.
One of the major improvements Irish Rail made was enforcing a reservation only policy on extra trains
Anyone who shows on on all Ireland Sunday without a reservation is either going to stand or will be left behind, no sympathy.