5 Adults - Rent a Large SUV/Van or 2 cars

It's probably driven by luggage more than people. If everyone has just a carry-on, a "compact SUV" might fit. If your idea of what to visit is varied, two cars offer some diversity - some folks can go to the art gallery while others go to the Neolithic ruins. We traveled with five in Scotland, and the decision was taken out of my hands because my nephew insisted he wanted the freedom of a separate car. We just met up for meals and places of common interest.

There's a couple of other things to consider:
1) Do you have two willing drivers?
2) An SUV in Ireland is nothing like an SUV in the States - much smaller.
3) Two small cars may actually be cheaper than a single large option.
4) Being squeezed a bit for space can usually be tolerated for what are pretty short drives if you have things to do and see along the way. Luggage in you lap is probably asking too much.
5) Does "large" mean tall or broad?

We've never rented in Ireland, but is Scotland several years ago we were given a small SUV and quickly discovered that 4 carry-on bags would not fit in the back. We were moved to an estate car that worked for 4 adults.

I can't imagine 5 adults being comfortable in a compact SUV even without the luggage issue.

Perhaps a 9 passenger if your group will always be joined at the hip or 2 cars if you want flexibility.

Take a look at the rental websites such as My Irish Cousin for an idea of vehicle size and capacity.

Here is a discussion from earlier this year. Scroll down to see the experience of others with groups.

Car rental advice for a family of 5

We were 4 average size people with carry on luggage and personal bags. We rented cars in Ireland and Scotland and I was the driver. In Ireland we had a Toyota RAV 4 (compact SUV) and we were able to fit in it fine. No way it would have been comfortable for 5, nor would additional luggage fit. In Scotland the first compact SUV didn’t fit the luggage so they gave us a Volvo that was slightly larger. I don’t remember the model. Again, it wouldn’t have accommodated a 5th person.

In Ireland the roads are narrow and parking can be a challenge. If you think you might want 2 cars so people can go do different things I would consider that. If you’re just going to be following each other from place to place you might want to consider getting one larger vehicle, trying to avoid anything larger than necessary. That way you are only paying for fuel for one vehicle.

You will definitely be more comfortable in something like the Tourneo, but keep in mind the difficulty of maneuvering and parking a big rig in a city like Kilkenny, where the streets are very narrow. Maybe also in Galway, though I'm not familiar with it. You may want to stay outside the city center and use public transport. You could also visit Kilkenny as a day trip by train from Dublin. Then take the train to Galway and rent and return your car there, using the train or bus to get back to the airport. You don't need a car for Dublin, Kilkenny or Galway. Train tickets are very inexpensive if purchased in advance.
Is the whole trip a week? Or is the car rental part a week? If the whole trip is a week, you're trying to see too much.

I would say if you can stick to the larger N and M roads, an SUV/Van should be fine. We just helped our son move and rented a larger Peugot 3008. This was the largest car we've ever rented in Ireland. Overall it was fine but there were a few smaller roads where we needed to back up and pull over to let another car/bus/van pass. If you do go with 2 cars, the make/model we rented held a LOT of stuff--all of our son's belongings as well as our two hard-sided carryons. I cannot speak to parking a van but I would ask at your various lodgings about the ability to park this large of a vehicle. They should be able to knowledgeably answer the parking question for their area.

We did a great Gate 1 Travel 10 day tour of the Irish Republic last year. We were 15 people with our driver/tour guide in a small bus.
Our driver knew how to negotiate some of the narrow roads and we had not issues. There were a few empty seats in the vehicle.

I have never rented in Ireland, but would prefer one vehicle, for 5 with luggage.
When we rented in the UK some years ago, I reserved a medium sized car for wife and self, we had two air checked suitcases and small briefcase/carryons that would NOT fit in the trunk, had to upgrade to a larger vehicle.

We're currently traveling around Ireland, and there are four of us sharing a Skoda Kodiaq as our rental car. It's still a manageable size to drive and would fit 5 people in terms of seating and trunk space. The rear bench seat slides forward and back, so you can choose to prioritize passenger legroom or cargo space.
The car is classified as Category IVAR with Enterprise.

Driving in Galway is a mess. Parking the car at Dyke Road Car Park (€10 per day) and walk from there worked our well for us.