If you book the trains ahead you get much cheaper fares.
For travelling on full price fares these two rail passes may be of use (buy at your first station if it is Athlone, Balbriggan, Bray, Carlow, Cobh, Dublin Connolly, Cork, Drogheda, Dun Laoghaire, Dundalk, Galway, Greystones, Dublin Heuston, Howth, Kildare, Kilkenny, Killarney, Killester, Limerick, Limerick Junction, Malahide, Mallow, Maynooth, Mullingar, Newbridge, Dublin Pearse, Portarlington, Portlaoise, Sligo, Thurles, Tralee, Tullamore, Waterford, Westport and Wexford ).
Trekker ticket- 4 consecutive days of travel €88
Irish Rail Explorer 5 days travel out of 15- €128
Full price fares are-
Dublin to Kilkenny €21.65
Kilkenny to Waterford €11.65
Waterford to Cork €27.20
Cork to Killarney €20.05
Killarney to Galway €44.90 via Limerick
Galway to Dublin €30.45