Is there an upper age limit for car rental in Ireland? I've heard it is 70 or 75.
No one else has responded yet, so I'll jump in - this doesn't actually answer for Ireland, but for Scotland last year there were age restrictions (both on the young and old ends of the spectrum) regarding car rentals. I believe this could vary from rental company to rental company, so it could pay to shop around, if any restrictions are encountered.
If you click here:
https://www.ricksteves.com/travel-tips/transportation/rental-car-requirements
you'll see that Rick states that:
"Drivers over 70 may have trouble renting in the Czech Republic, Great Britain, Greece, Northern Ireland, Poland, Slovakia, Slovenia, and Turkey."
Note that he says Northern Ireland.
And see this from TripAdvisor:
http://www.tripadvisor.com/ShowTopic-g186591-i88-k4293424-Car_Rental_for_Senior_Citizen-Ireland.html