The neolithic passage tombs north of Dublin, Ireland were built around 3000 B.C. (about 1000 years before the stones were erected at Stonehenge, and 500 years in advance of the great pyramid at Giza, Egypt). The most famous is Newgrange. There is a long passage under the mound (the entrance is in the center of this picture) leading back to the center. It is thought that important people were cremated after death in the small chamber at the end of the passage (although, to be honest, very little is known with certainty about these monuments). On the winter solstice, December 21, a shaft of light penetrates from a specially constructed window above the entrance back into the passage, one of the earliest known examples of a large monument with an astronomical alignment.  (photo by G. Rieke)

To the far right is the light streaming through the window on December 21, 2003 (photo by Fran Caffrey). To the right is a map of the passage; the entrance is at the bottom and the ceremonial chamber under the center of the mound is at the top. The yellow line is the path of the sun light on the winter solstice. (from Mythical Ireland, http://www.mythicalireland.com/ancientsites/newgrange/illumination.html)