I saw where someone on the forum talked about starting from Dublin, visiting Glendalough and then driving to the Rock of Cashel, and then down to Waterford, I think. I realize this would be a very full day but what roads would a person take to cover that itinerary? Looking at maps, it's not clear to me the best roads to drive .
I have just answered this in the other Irish Road Trip thread: Dublin - Glendalough - Hollywood - Ballymore Eustace - Naas - Cashel.
The road from Cashel to Waterford isn't as good, but it is a fairly straightforward drive from Cashel to Clonmel and then Carrick-on-Suir and on to Waterford - effectively following the River Suir all the way to the sea. It should take about 1hr 15 minutes, but it will seem like a very long final leg on a long day's driving.