We will be renting a car for 4 adult women in Dublin in September, and continue to be mystified by what exactly is the standard luggage size (or bag as they call it in Ireland) when figuring what will fit into the car. I have 'googled' everything and cannot seem to find the answer to this.
Irish cars are small. With four people, you'll need a larger car even if each has only one "carryon" size bag. My son and I each had a 21" bag and a duffel, and we rented a subcompact that was supposed to be able to carry three bags. Only one of the 21" bags fit in the trunk, along with the duffels. The other bag had to ride in the back seat. Make sure you are comfortable with your choice and upgrade if necessary. You don't want your trip to be marred by being cramped in a car the whole time.
Find out what make and model you are being offered and we can tell you if your luggage will fit without causing discomfort to the four of you. It may help if you can post the approximate measurements of your cases ( that's another word we use ).
Please remember that Irish roads are often narrow and full of bends. A small car can get you out of big trouble but an overcrowded car can create trouble - it's a dilema.
The following example is only a point of reference since it will be too small for you. Three 21" carry-on wheeled bags will fit--barely--standing upright side by side in the back of a Ford Focus hatchback. That worked fine for the three people in my party. We used the empty passnger seat for our daypacks.
We rented a Nissan Primera.It seated 4 adults very nicely and we were able to get 4 small suitcases plus a few carry on size bags in the trunk.
Kim is right: in the UK/England, and I'm guessing also in Ireland, the smallest (narrowest) car that will accomodate your luggage (sorry Al, cases) = good, wider car = not good. It would be easy for an uninitiated American to fail to appreciate the narrowness of some of the roads over there. Before driving the roads in the Cotswolds in 2005, I had noticed that the Ordanance Survey map I had purchased ahead of time distinguished between "roads generally more than 4 meters wide" and "roads generally less than 4 meters wide," and I thought to myself, well that's a bit odd, how many paved roads would be less than 4 meters wide? The answer is that a fair number of roads were less than 4 meters wide, and many of them had a stone wall on both sides.
I rented a care in the UK last summer. It was 4 adults with quite a bit of luggage. We rented a station wagon which was great, but also rather expensive. We did a lot of driving, so having a larger car made things much more comfortable.