type of car to rent, and thoughts on renting a Nav unit?

It was years ago, but we probably had something on the scale of a smaller sedan, and that was fine. I just looked at National's website, and it looks like you can get a smaller Nissan SUV that would do. Just needing an automatic will limit the options pretty quickly, try a couple of sites and see what is available. In the end, I would pick comfort over worrying about narrow roads, even their larger vehicles are pretty nimble.

As for navigation, if you will not have a smartphone with data, then navigation is worth it if you will be in more rural areas, if sticking to the more well traveled parts, then maybe can do without, using a map and signs.

You did not mention, but be aware that your Credit Card Rental Insurance likely is not valid in Ireland, leaving you to purchase CDW and Liability from the rental agency, so consider that in your budget.

The main factors you need to consider is how much luggage you'll have and the limited selection of automatics.

If opting for an automatic you'll be looking mainly at the higher end of the German luxury vehicles and they can be relatively expensive. Something like a VW Golf would be adequate but possibly could be difficult finding one in automatic. Where will you be renting from? Airports often offer the greatest selection of vehicles.

It's also worth bearing in mind that most new cars are being equipped with Satnav as standard. The last few cars I've rented in Europe have come with it equipped, an Opel Astra, Renault Clio and Opel Moko to name three reasonably cheaper rentals. Every Mercedes, BMW and Audi I've rented has come equipped with it as well. It's highly likely that you'll have a smartphone and Google maps is a perfectly acceptable navigation tool. Car rental Satnav's are an absolute rip off, I would never pay for one.

Related to this, make sure you review the car seat laws in Ireland: http://www.rsa.ie/Documents/Road%20Safety/Safety%20for%20kids/Child%20Safety%20in%20Cars%20English.pdf
I'm not sure what they are where you live, but here in CT children are required to be in booster until they are 8 years old or 60 pounds - while Ireland requires a booster for children up to 79 pounds. I remember another post some time back in which another family's 11 year old was quite miffed to learn she'd have to use a car booster seat again!

Thanks everyone! Yes, I'm aware we'll be required to get CDW. Another pricey addition is adding a second driver. I did NOT know my (skinny) 11 year old will need a booster though! He will be devastated.... That's also pricey depending on the company, so knowing we need 2 means we might just buy them there and jettison them as we leave. I appreciate the advice that its okay to go with a sedan vs economy to have a little more room.

I don't know if you have a GPS at home or not but I have a garmin in my home car and it has a chip slot for map updates. I purchased a UK/Ireland mini disc on ebay for around $12 and installed it on my unit (Garmin had it for $79). I took my GPS along and it worked very well. One thing I noticed on my GPS is there were a lot of roads out in the back country areas that did not have road names or numbers. But being able to see the unnamed roads on the GPS made it pretty easy to navigate. I also am very familiar with my GPS and its operation and that was a great plus when trying to navigate round west Ireland.

We just rented a "hot spot" to allow us to have internet in and out of the car. Then we used our google maps app for directions, this is necessary because the signage is not at all like the signage in the USA. With the hot spot you can check lots of things ahead of time, such booking admission tickets, restaurant times and menus, info on attractions, etc. A GPS will only give directions.

Thanks for info on the hot spot! I made our rental reservation this week and splurged on a hot spot since it can be useful in and out of the car. For anyone else needing a car seat or booster (or two!), I did learn that some rental companies charge a seat rental by the day, but Hertz rents it for a set one time fee. Much more affordable than the daily rental fee at other companies, which more than offset the slightly higher rate at Hertz.