From the Budget (Ireland) website...
"Customers over 75 years are eligible to rent with the following specific conditions:
They must drive regularly
They must provide us with a letter from their insurance company proving that they hold a current motor insurance policy and that they have been driving without having an accident for the last 5 years.
They must provide us with a letter from their doctor stating that they are in good health.
They may be required to undergo a driving assessment accompanied with a Budget representative."
http://www.budget.ie/terms/republic-of-ireland/