We'll be dropping the rental car in Kilkenny and taking the train to Dublin. Which station in Dublin is closest to Harding Hotel?
Trains from Kilkenny all go to Dublin Heuston station. From there, you might take a taxi to the hotel (easiest), or bus #90 (which runs along your side of the river), or LUAS Red Line, which stops along the opposite side of the river (see T stops on Rick's map, p. 106 of Ireland book). Train tickets are cheaper when purchase online at http://www.irishrail.ie/, even on the same day of travel.