We went to Ireland and England last summer. Loved it! Want to go back!
One thing I found was to go to the UK or Irish websites instead of the American websites. For instance, Premier Inn offers specials on it's Uk site that are not offered on it's american site. Hertz had better prices on it's UK site. I would type in something like "Hertz in UK" when searching. I also had found a code for Hertz that was good in Europe. Did a quick search but didn't find it right now. Most car rental codes I found only worked in the US.
I signed up for emails from Premier Inn also. It's a budget hotel, but every one we stayed in was very clean and fairly new. I can post some pictures if you're interested. They alert you to sales like this one for 29 pounds (or euros, depending which country) a night:
http://www.premierinn.com/en/it/offers/premier_offers.html?DCMP=EMC-alert_25feb10rmngen
We rented a car (stick) and dh did all the driving. He swears he won't drive there again. Driving has never bothered him before, but I wonder if the fact that he is in his 50s now is part of not liking to drive there.
Definitely get a GPS
and a map. We went with just the GPS but wished we had a map too. The roads are often unmarked, plus we would drive thru towns and never even know what the name of the town was! No signs with the towns name on it. With the roads being so narrow, you do not want a large car. Parking can be very difficult, with narrow spots. I can't imagine trying to park and drive anything larger than the small car we had! Speed limit on many roads is only about 40 mph or less, plus the roads are winding, so it takes a while to travel. Major roads are better, but you often need to get off them to get to where you are going.
We stayed at the Best Western Premier Academy Plaza Hotel in Dublin. It was a bit more expensive, but it was nice and located near O'Connell street, a major tourist area. The sightseeing bus stops right across the street and the airport shuttle has a stop close by on O'Connell. The only downside was that it appeared to be a couple of older buildings connected together, but I only noticed this while walking from the elevator to the room. Our rooms were wonderful and staff was very helpful!
Probably our number one attraction was Giant's Causeway on the north Antrim coast. Absolutely stunning! Plus it was a nice break from touring buildings and cities.
http://www.bing.com/images/search?q=Giant's+Causeway&FORM=IGRE#
Just north of Dublin, there are some attractions in the Boyne area that we did on our way to the Antrim coast. Bru na Boinne, a megalithic chamber tomb older than the pyramids.
http://en.wikipedia.org/wiki/Brú_na_Bóinne
Monasterboice has three Irish High Crosses, two of which are said to be the tallest and best in Ireland.
http://www.sacred-destinations.com/ireland/monasterboice.
Mellifont Abbey is a ruined 12th-century Cistercian monastery near Monasterboice in County Louth.
http://www.sacred-destinations.com/ireland/mellifont-abbey
Also there is the Hill of Tara and Trim Castle, which we did not get to.
I have an active teen who wanted to be outside and moving about, so that is why I chose some of these things. At the Abbey, Monasterboice, and Giant's Causeway, he could move around at will and be active. We did tour museums and historic sites in Dublin, but he got tired of seeing "old things".
We also visited Peatlands Park just south of Lough Neagh (the lake near Belfast. We enjoyed it, very peaceful, but I wouldn't go out of my way to see it. The website said they had peat-cutting demos (I wanted to see peat) but they only do it for arranged school groups.
http://www.bbc.co.uk/naturescalendar/autumn/wetlands/peatlands/peatlands_access.shtml