It's great to see that you and your whole family will be travelling around the Emerald Isle this March! The right type of vehicle is indeed very important, and I'd love to provide you with more information.
Coincidentally, Gary recently had the same question but was looking for a car for six passengers and their luggage. You can find the thread with more helpful information and tips here
In this case I recommend to reach out to some of the approved car rental companies for suggestions, as they will know the sizes of the vehicles they rent out. Take a look at the Car Rental thread
and Vehicle Rental page
for more helpful information.
Most places have scenic routes, where it's recommended not to drive with larger vehicles, like for example campervans. These roads will usually have signs indicating this. Are there any roads or routes that you are planning on taking that you would like to know more about? Most of them will fit a larger to normal-sized car without issues.
I hope some of our other members will provide you with their tips and experiences as well! :)