It is correct that the €10 fare (and all web fares) can only be purchased up to 90 minutes before departure. As an internet-only product, the €10 fare cannot be purchased at Heuston booking office at any stage. You will note from
http://www.irishrail.ie/index.jsp?p=118&n=197&a=265 that the standard booking office fare for Heuston to Kilkenny is €24.00 (fare band K).
I had thought that €10 fares required to be purchased the day before travel at the latest, but that may have changed or have never been right.