I am travelling to Northern Ireland and Dublin this summer with a friend. We both want to see a castle but probably not more than one. We will be in Belfast, Derry and Donegal County with a car. We will also be spending a few days in Dublin without a car. We are not interested in looking at ruins of a castle but rather an actual castle that has been preserved/restored. The castles I've read about are: Trim, Dublin Castle, Glenveagh & Carrickfergus. Any thoughts on the best of these or suggestions of others in the area?
Northern Ireland mostly has rough ruins or things that are more like manor houses. Carrickfergus (Norman) is about as good as you'll be able to find that's in fair shape and looks like what most people think a castle should.
Trim has had restoration work, but is still pretty ruined. Many of the restored castles, as Ed says, are really more like manor houses or mansions.
I thoroughly enjoyed visiting Trim. The tour guide for the interior part does a good job of explaining the different periods of the castle's occupation. Dunluce is also fun, and it's close to the Giant's Causeway.
Dublin Castle is not really a castle so much as a complex; there was once a castle there but all that remain is a tower from the 13th century amid buildings from the 1800s - 1900s. I think I'd recommend Carrickfergus, it is not far at all from Belfast. And as a side note, I just got back from a trip to Northern Ireland and you will love Derry - it was a fantastic city and I can't wait to go back!
Trim Castle is the largest Anglo-Norman castle in Ireland. I found it very interesting and there is a great view on top for pictures. The guides there are very personable and informative.