Hello Bob,
It's great that you are considering a road-trip to discover Ireland!
In some cases, car rental companies require drivers over 75 to provide a printed medical certificate which states that they are fit to drive. Other travelers had a similar question, which you can find on
this thread.
We hope to welcome you here when the time is right. In the meantime, I'd love to hear more about the itinerary you had in mind for your road-trip.
Cheers,