Dublin-Kilkenny-Rock of Cashel-Cobh-Kinsale

You'll be quite rushed if you try to visit all those destinations in the same day. If I were you, I'd break up this part of your itinerary into at least two days with an overnight, perhaps in the Cashel area.

By the time you visited Kilkenny and the Rock of Cashel anything you might want to do in Cobh will be closed or closing. You don't mention where else you plan to go and how much time you have but could you go straight to Kinsale from the Rock of Cashel and do Cobh as a day trip from there?

I realize that it is very rush. I wish that we had one extra day.
So if we drop Cobh, is Dubin-Kilkenny-Rock of Cashel-Kinsale manageable?
Alternative to a B&B/hotel in Kinsale, we can also stay outskirt of Cork. There seem to be more options that are still available.

I suppose the answer to your follow up question is, what do you want to do? I assumed that Cobh was of interest because of its Titanic connections. Kinsale is about the Charles Fort, lovely walks around the town and along the water and food. You could certainly do the last two in the early evening and evening.

One reason you'll be pinched for time is that you're electing to collect your rental car in Dublin city where the offices don't open until 9 AM or so. If you were able to change your pickup to one of the airport locations you could get a much earlier start to your day - the concessions there open early, ie around 6 AM.

Thanks for the replies!
Yes, we are interested in the Titanic experience in Cobh. So if the time is tight, we will skip Cobh.
For Kinsale, we will spend more time on the next day.
The car rental where we have reservation is in walking distance from the hotel in Dublin centre. We will drop off at the airport at the end of our trip.