6 person car rental for 2 weeks

I don’t know anything specific to Ireland but my experience in Italy and Greece is that it is cheaper to rent two vehicles than one large vehicles.

Contact gemut.com for quotes. They helped me with my rental in fall 2021 in continental Europe.

I have no idea about Ireland, but for a similar situation in Iceland in December, I found that renting 2 small vehicles 3 to 4 persons each, worked out much better for us financially. If you have more than one willing driver, this might work? I took a Hotspot option for one of the cars, and we synced our phones in case we needed to contact each other while driving the Golden Circle.

Rental car prices are high everywhere right now - Ireland is no exception.
As Beth mentioned it will be much cheaper to rent two mid-size vehicles rather than one van. Besides, with 6 people and their luggage you're going to need more space than you think.
One workaround that might help reduce the cost of the rental(s) is to make what use you can of Ireland's public transportation system. You can, for example, catch the train or bus from Dublin to most of the major cities and towns and then maybe only have to rent for a few days in order to explore the countryside versus for the entire 3 week period.
One-way drop charges typically aren't too bad in Ireland, so you might want to consider doing that rather than booking a round trip extended loop from Dublin.

I don't think it was as bad as in the English Lake District, but I think that local roads in Ireland are not as wide as you are used to. Combined with driving "on the wrong side", it will be a difficult task for your more local driving.

Have you tried pricing directly with rental companies, as well as Travelocity like sites? Have you (just priced, at least) deals you don't want, say arrival airport to arrival airport? Make sure your credit card covers Collision for rentals in Ireland, some may not. And remember to bring proof of your coverage to show the rental desk, it's demanded nowadays.