We have the simple explanation here
http://www.platform11.org/passenger_...discount_cards
I have seen notices at stations in the last year and it is in the back of the printed timetable
It took IE a few weeks to invent the excuse but it goes like this
There are two classes of route, the double track, frequent, reliable and the single track slow unreliable. So Tralee, Cork, Limerick and Belfast fall under the fast and frequent while Galway, Mayo, Sligo, Waterford and Rosslare fall into the slow single line camp, different cost per mile applies depending on the route you travel. Now thats reasonable enough, the discount level is murky and seems to be driven by business factors to maxmise revenues.
Euro standard practice is to offer a fixed discount of 25% or even 50%, but Irish Rail are far from doing things the industry standard way.
The first issue at hand is not the discount per say its the slightly werid pricing of certain journeys. There be two fares Bray Belfast, 15.50 the difference same trains same day same booking office but different fares, it depends on the intelligence or lack thereof of the clerk on duty. There are adult fares which are cheaper than the discounted ones
Your next trick is to buy a Dublin City Centre to Limerick return ticket which is 2.40 more but valid on the Luas and 90/91/92 to reach Heuston from the city