As noted above, your choice is much restricted if you are going to use a debit card. And taking the additional insurance is usually mandatory, but if it is only a one day hire that shouldn't be punitive.
Check age restrictions. If you're young, that can add more restrictions (and cost) and some companies won't rent if you are over 70. Have you had a driving licence for more than a year - you'll have trouble if you are recently qualified.
I recommend hiring directly with the company, not via a broker. I see so many horror stories with brokers - details and prices get mixed in the comms, or the booking lost entirely. If you have an email confirmation from the company itself you are much less likely to hit any hitches.
Don't expect to get the car they quote - they are only "examples" - all you can expect is something similar. If you want/need an automatic model, you must specifically book that type of vehicle, or you might get a manual. That said, manual gearboxes are becoming rarer, even on hire fleets. I rent a few times a year and the last time I was given a manual transmission car was over a year ago.
Take your driving licence, your card (the physical one, not a virtual one in your phone) and your passport. I rented a car on Wednesday this week and I witnessed one guy turned away because he didn't bring his licence and another refused for not having the physical payment card.
Expect to spend the first hour screaming in frustration as you try to work out how to turn off the hateful ADAS features (lane keeping, emergency braking alerts for a pedestrian about a half mile away, speed limit warnings that will read the wrong signs about 50% of the time).
Work out where you will refuel the car when you return - letting the hire company refuel it will cost an arm and a leg.
If there is the option to check in advance (it varies by company), do that - it will save a huge amount of time at pick up.
Some of the motorway roads near Dublin airport are toll,, but no barriers. You need to pay online. Like the Blackwall tunnel or Dartford Crossing. The hire company won't cover this and if you fail to pay you'll pick up admin charges from the hire company and a fine.
Enjoy the day