There are no train suplements that i know of.
Ireland is a small country with really only 3 major cities. Dubin, Belfast and Cork.
Dublin is central and all the train networks go to dublin. If you wanna hit the medium sized towns and the 3 cities the rail network should be ok.
A lot of people come to ireland to see the west, the best bet would be to get a train from Dublin to Sligo.
Dubin to sligo - About 2 hours by train.
Dublin to Belfast - Around the same.
Dublin to Cork - Just under 3 hours but there's some nice scenery along the way.
I know this is a year old but its good to have the info up.