Make sure you have all of your insurance lined up for Ireland. Both NI and Republic. That is the major issue with Ireland Car rentals. Hertz and Enterprise are more forgiving but the others especially Sixt and Dooley are very specific on the documentation they require. Some require recently documents letters on company letterhead stating you are covered the the class of car renting and the time period you are in Ireland.

That is if you aren't taking their insurance, if you are, then you are fine, just make sure you add in those costs. They aren't cheap.

Also make sure you check the fuel return. Some want a print out receipt within 5km of the airport or they charge you a fine.

If you are looking at company websites, check both the .com and .ie endings. I often find that the .ie endings are cheaper and I've never had a problem with the rental and I've done it twice a year for the last 15+ years.