It's complex, and dating says it's 4000 BC and younger; the passage graves have genuine boulders propped on supports that look eerily like the European neolithic monuments; as a bonus, there are carvings both on the site and in the visitor's center, claiming to be Ogham and sun-god related. There is even a complex "altar" stone with an underground whisper chamber beneath it.
Less convincing were the smaller "standing stones" and the astronomical alignment theories. We saw a lot of stones that were identical to the stones claimed to be alignment markers, which weren't labelled in any way. Amateur archaeology could be responsible for any number of bad theories and moved stones... but that said, an awful lot of it looked very authentic indeed.
Some of the sites on the topic: pictures of the stone writing, pictures of the site monuments, a timeline of the site's major datings and known historical events, a not bad short article on it, describing the altar stone. Here's the official visitor's site. (They keep pet alpacas there too, who are very pretty.)
All in all, a recommended visit!