Itinerary questions

We're planning a similar trip... While I'm still trying to decide how to do the Dublin to Dingle leg it seems like Cashel would be no problem as a stop; I believe it's about half way between and most seem to spend between 1 and 2 hours there. As for Kilkenny I actually just posted another topic asking how much time one needs there. I'm relatively sure if you wanted to see maybe one thing in town and get lunch it would be doable with Cashel on day 3 for a late afternoon arrival in Dingle but as I said before, I'm still trying to decide whether such a brief visit is worthwhile. I think Glendalough would be too much based on the research I've done on travel times between sites and how much time you would need there.

The other option (this is sort of what I'm considering) might be to go to Kilkenny on in the morning of your day 3 before going to Cashel in the afternoon and either sleeping in Cashel or Kilkenny that night before heading to Dingle on your day 4 . You could maybe even fit Glendalough in on the morning of day 3 before going to Kilkenny (perhaps by doing those 2 on day 3 and hitting Cashel on day 4 on your way to Dingle).

Greetings from Ireland.
The key thing to remember is Ireland is the size of West Virginia (Rick says so, so it must be true!), I have often drive from Belfast to Dingle (far north to deep south) in one day. An awful 7 hour drive that I wouldnt recommend, but perfectly doable. And I will go from Galway to Dublin (West-East) tomorrow in less than 3 hours.
The question is, how tired do you want to be, is it quantity or quality you want. And both are equally valid. Often when in North America I want to see as much as I possibly can on a given trip. But there are downsides of course, the '3 Fs' - fatigue frustration fights!
A day trip to Killarney from Dingle is a piece of cake, and worth it in my opinion, but why go back to Dingle then, you'd be better off staying in Killarney that night. Or after you visit Killarney head from Killarney to Limerick on the 'good' road and stay in Doolin, Ennis or Galway even.
Doolin is a nice spot if you like trad music, hikes and a wee bit of silence. Galway is way livelier at night. I find the young and young-at-heart prefer Galway. I've just spent 3 nights in Doolin and 3 nights in Galway (typing this from the latter). I visited the Aran Islands from Doolin on Saturday with Ricks son, Andy, and we had a great time. Service wasn't on time, frustratingly, but was reliable; although I thanked God there was a calm sea for otherwise the little boat we went to Inis Oirr aboard wouldve been a mildly traumatic experience! :)
The ferries from Rossaveal are big comfortable boats, but it is an hours drive out from Galway to get to them.
Kilkenny is always a good overnight, if you can fit it in then do - nice people, good history, looks great. One night is enough though I think and if you cant overnight there, your Ireland trip wont be any the less for it. The other thing is, people enjoy the Round Tower, Celtic Crosses and Monastic spirituality of Glendalough. They enjoy the very same about Cashel. Both sites have a similar theme. If you can fit in both, do. If you cant, don't beat yourself up. Come back in the future for another relaxing holiday!
I could go on forever so Ill stop there. Let me know if you have any more specific questions.
Slainte/Good health
Stephen McPhilemy
Rick Steves ireland Tour-Guide
Dingle and Derry

Thanks so much for the valuable advice! I'm inclined to agree that trying to reach Glendalough plus Cashel and Kilkenny would be too rushed--unless we spent a night in the Kilkenny area. I want to explore as much as possible, but I also want to "take it as it comes" and allow for unforeseen adventures. So much to balance in a 9 day trip!

I'm looking forward to exciting car and boat rides. I did spend a few nights in Doolin about ten years ago and enjoyed the quiet town. I emailed four different b&bs there this week to check on availability the first week in May and all replied that they're booked, so I'm a little afraid it may be more touristy there now than it was a decade ago?

I would skip Glendalough and stick with the Rock of Cashel if you are driving to Dingle. The Cashel to Dingle route in a day is totally doable. Glendalough takes more time to enjoy and doesn't quite fit on the path you are going.