The Brugh na Bóinne (Boyne Valley) of Ireland has several large metalithic tomb sites, including Newgrange, Knowth, Dowth, and Four Knocks; together they rate as #4 on our all-time greatest megalithic sites to visit.
For basic information on the Boyne Valley Megalithic sites, you can't do better than Knowth.com; take a look at the Heritage of Ireland site for how to get there.
Lots of information can be found on the Knowth site; and you can also find some fabulous photographs from the Stones Site of Paola Arosio and Diego Meozzi. Also well worth a look is Tom Bullock's CD of Stone Circles.
The Mythical Ireland site has a news server reporting on several relatively recent investigations in the Boyne Valley.
Meanings and Interpretations
Some of the more recent research in the Bru na Boinne has been trying to determine if there are astronomical alignments associated with the passage tombs. Here's a recent report.