We rented our car this summer through e-car rentals Ireland, they're a consolidator, the car was through Thrifty. For a compact car with the basic CDW we paid about 150 euro for the week.
The basic CDW comes with a 1100 euro deductible. To cover that, you can either opt for the super CDW through the rental company, which will about double the cost of the rental, or, as was suggested before, go through insurance2carhire (you'll actually go to another website, dailyexcess.com). We did that and paid about $60 for the extra coverage.
We did get a HUGE scratch down the side of the car. So here's what the rental company did -- we were charged the 1100 euro, since our additional coverage was through another company, and we had to file a claim through them. We had a copy of our policy with us, so we knew what paperwork we needed to have. It took about a month to get the money from the insurance company.
I have a World Mastercard, but was nervous about getting coverage through them, as they didn't cover liability.
As far as accomodations go, you should be okay just finding rooms as you go, but I would book your place in Dublin before you go. Then you can drop your rental car off at the airport and go directly to where you are staying, and know how you're going to get there, taxi or bus.
I think trying to do Bunratty, the Burren, and the Cliffs is a lot for one day. I'd cut out either the Burren or the Cliffs, get a place to stay in Doolin or somewhere close by, and go see whatever you missed in the morning.