A week ago we visited Brú na Bóinne while Christie was in town. While there are several sites in the area, we only visited the top two on this visit, Newgrange and Knowth. These are earthen tombs from about 1200 years ago. When you show up, you reserve a bus ride and a tour out to each site. To be honest, for all the hassle that might go in to getting a tour and what you actually see, it was just ok. Going in the Newgrange tomb and seeing how they built it to align with the solstice it neat, but that is the only thing there really.
Pictures can be found here.