Dooley Car Rentals have no maximum age limit.
This is their policy on the use of debit cards:
"We also accept most debit cards as a form of payment. However, if you are using a debit card you are obliged to purchase the excess insurance and a deposit of €500 will be required in addition to the rental charges. A reduced damage excess of €100 will apply (included in the €500 deposit). When the car is returned the deposit will be refunded"
Note that Dooley (and probably other Irish car rental companies too) require drivers with UK licences to have the relevant code so that they can check how many points there are on the licence:
https://www.gov.uk/government/news/hiring-a-vehicle
https://www.dan-dooley.ie/