Ireland

I agree that you should book ahead. I have had good results booking with Autoeurope in seven or eight different countries, including Ireland.

Book now, especially if you want an automatic. Then you can keep watching the prices as the trip approaches, and change bookings if you see a better deal. There is no advantage to waiting. You will also want Super CDW insurance to give you a $0 deductible in case of an accident.

Do you recommend a certain website for reserving a car, or would I be better off doing a deal on one of the various websites that include flight and car?

I have used Irish Car Rentals in the past, but I would go with whichever gives me the best price. A flight/car deal might be better or might not - and it might make you less flexible if you want to change your rental later. Autoeurope is a consolidator that uses lots of different companies, so that might save you some individual checking around. You will just have to run the numbers yourself to see what is the best deal.

Booking ahead makes sense, but not for the day you arrive in Dublin - book it for the day you plan to leave Dublin.

It makes a lot of sense to rent your car well in advance but continue to look for specials as your travel date gets closer. I suggest picking your car up at the Dublin airport and then finishing your trip actually in Dublin. You could either bring the car back to the airport or drop it off somewhere else and bus or train into Dublin. You really need to plan what you want to see. The airport bus is very convenient into/out of Dublin. On your first day you could see Newgrange and Knowth which are just north of the airport. There are other attracting in the Boyne Vally. Trim Castle was also nice.