We rented a car to get from Belfast to Dublin from National, the rates were nearly identical between the companies we looked at.
I generally use carrentals.com, they will give you pricing for all of the rental car companies except for Enterprise usually.
My best advice, unless you have driven in Ireland before spend an extra $10 day and get the GPS as well. Also learn about how roundabouts work (if you do not know already)or you will be reenacting one of my favorite scenes from National Lampoon's European Vacation where the Family Griswold is stuck driving in circles for hours because they can't figure how or where to exit the roundabout.
D/H drove and I was the navigator, my job was to let him know when a roundabout was coming up and to count the number of exits so he'd know which one to get off on
. One of the highlights of that trip for d/h was getting to drive on the "wrong" side of the road. He had a ball. Driving in Dublin was almost as challenging as trying to find a parking spot in Dublin
. Make sure if you are staying in Dublin that your hotel offers parking or has it nearby... from the voice of experience lol.
Oh and get the zero$$$$ deductible insurance waiver, just in case