Senior citizens getting around in Ireland

Car rental ages are dependent on the specific car rental companies. They will not all be the same as far as age requirements.

This Link has a pretty comprehensive, company by company, discussion of policies regarding renting vehicles to seniors over 75 years of age.
AutoEurope is quite helpful in sorting things out too - just deselect the "Driver age 25 to 69" box at the bottom of their reservations page, insert your own age and see what pops up. There are typically at least a couple of companies that rent to seniors without any restrictions.
Hertz is one of the more restrictive companies but they do allow senior rentals per the following:
"Senior Driver Policy: Customers over 75 years are eligible to rent with the following specific conditions
-You must drive on a regular basis;
-You will need to provide to the counter a letter from your insurance company to state that you have not had an accident within the last 5 years, that you hold a current policy of motor insurance with you and that you are currently driving; -You will need to provide a current letter from your doctor to state you have been in good health."
Best to speak directly with a live human being to sort things out before you show up at the counter.
Bottom line is that renting a car as a senior in Ireland is eminently doable but will require some research beforehand, and I would add that touring by car is by far the best way to see the country.

The members of the Car Rental Council of Ireland (all the major car rental companies in Ireland) do not apply an upper age limit to their customers. We are aware of some brokers and third-party websites selling car rental that refuse over 75s, but car rental companies have no control over these sites or their policies.

While the exact requirements will vary, all car rental companies may require additional information from older drivers intending to rent a car. This is at the insistence of their insurers and is consistent with the approach of the State, which insists that “A medical report form is required when applying for a driving licence or learner permit where the driver is aged 75 years and over”. (Ref: National Driver Licence Service website).