Our trip to Ireland was for just shy of a month. We only visited the west coast. We stayed 10 days in Dunkineely, County Donegal - Wild Rose Cottage:
http://www.tripadvisor.com/VacationRentalReview-g551497-d4260699-Wild_Rose_Cottage-Dunkineely_County_Donegal.html (You'll need to copy and paste since it didn't show up as a link to click on)
We rented a VW Goff (diesel) and an automatic. I highly recommend renting anything small, diesel and automatic. You'll pay more for the automatic but, trust me, you'll be able to breathe. I think Americans get into trouble because they want to rent bigger cars...like we are use to here in the U.S. and these bigger cars don't work well on the smaller, Irish roads. Not to mention driving through towns where cars are parked half on - half off the road. You'll be sucking lots of air. A small car, however, doesn't have lots of room for big suitcases. We had two 26" cases and two tote bags. Only one suitcase and one tote bag went in the "trunk". The other suitcase and tote bag was in the back seat. If you were travelling with another couple this would be quite a shock when you realize it's not all going to fit. You need to do your homework on what car to rent if you are travelling with another couple. And, take it from me. Take out half of what you originally packed. You don't need it.
We rented our car through Enterprise and picked it up upon our arrival at Shannon Airport. Service was fast and courteous. We also took all the insurance that was offered. I had read and re-read comments on TripAdvisor's Ireland Forum regarding insurance. Most of the time I would be more confused after reading them; however, we came prepared with documentation from our credit card company showing they would provide coverage through our card (Marriott Platinum Visa). But, in the end, we decided to bite the bullet and take all the coverage through Enterprise. We were going to be in Ireland for almost a month and, bottom line, we didn't want to worry about who or what to call in case something happened. Or, hassle with our credit card company over "proper" paperwork for reimbursement. Everything was covered including windshield replacement and tires as well as allowing for another driver...me...fat chance. There's no way I could drive and breathe at the same time. It was pricey but I would do it again for the peace of mind.
The month of May was (according to Irish news) the coldest and rainiest in 120 years. I think we had maybe 4 days of cumulative sunshine. It was cold, windy and raining. It's a good thing we didn't go to Ireland for the weather! And, it's true. One can experience 4 seasons in one day. Dress in layers, bring a warm hat, scarf for the neck and a raincoat with a hood. We each had umbrellas but never used them because of the wind. We told ourselves we wanted to remember the weather when we were back home in Arizona and it was 110 degrees. Guess what? It's 112 today.