Can't say I agree with you. On the front page it says "Fares online from just €10", and clicking Fares and Tickets then Intercity Fares gives you
http://www.irishrail.ie/index.jsp?p=118&n=197
"Select your route below to view the range of tickets and fares
available at your station:"
And so on. I would say it's reasonable general knowledge that booking travel in advance online tends to be cheaper than turning up on the day.