Hello Henri and welcome to our Community!
I can reassure you: there is no upper age limit to rent a car in Ireland. However, some rental companies will ask you to meet requirements if you are above 75. Typically, they'll ask for a doctor’s letter or certificate of insurance but it depends on the renter's policy. I would suggest checking with a few rental companies directly—Enterprise, Hertz, or Irish Car Rentals are known to be flexible. Here's also
useful information on renting a car in Ireland.
Do you already have an itinerary in mind for your road trip?
Cheers,
Antoine