Jim,
Your latest idea sounds realistic and set up in a way to really enjoy your trip AND your family. Dingle Town is small but the area surrounding it is stunning and there are day trips to be had without the get up and go or your previous itinerary. Your son is correct in saying the North shouldn't be missed....so return one day to see it. And as another poster said 35 mph is generous regarding speed and top spped will be 60 on a major thoroughfare but who wants to see Ireland that way?
Enjoy