I have used Sixt twice in Dublin, and won't do it again. Both times we had one hour plus waits while all the other companies' counters were open. There also can be a wait at their drop-off location to shuttle back to the airport. The crowd waiting is often larger than the bus can handle and you need to wait for the next trip. Whoever you go with, ask if they include a spare tire. It sounds silly, but in England many companies don't and if you get a flat, you have to wait for road service, last summer in England we waited nearly five hours for the road service.
I'd go with the smallest car you can tolerate. The roads are narrow and many bends are blind as the road is bordered by stone walls and/or tall hedges. You'll be glad for the extra clearance room if you meet an iron elephant (tour bus) coming the other way.
Automatic is almost a necessity. I am very comfortable in a manual and usually prefer them, but not in Ireland. Shifting with the other hand while dealing with the narrow roads and driving on the other side is more than you will want to deal with. Unfortunately, it seems the rental companies realize this, and the price of an automatic is usually twice the cost of a manual. We rented a small Mercedes with auto in the UK for about $35/day, in Ireland we paid more than that for a Toyota Yaris. But I was glad to have that tiny clown car instead of something larger. Zero deductible will be expensive. 3rd party insurance isn't accepted in Ireland, and low deductibles will be pricey.
I found a private car/van to be good for getting to/from the airport. The cost is reasonable and you can schedule for when you need it. I have been very happy with http://www.dublinairporttaxis.com