Newgrange

"Older than both Stonehenge and the pyramids of Egypt, the monument at Newgrange was built around 3,200 B.C.E. Its use is a mystery, although it was most likely a place of worship, and there are legends that it was used as a burial chamber. The main circular mound has a passage with small chambers off it, and each year on the winter solstice, the sun travels along the passage and lights up the main chamber. Many of the curbstones at the front and stone slabs lining the passage have decorative examples of megalithic art, with zigzags, spirals, and other geometric designs. Access to Newgrange is by guided tour, and it’s part of the Brú na Bóinne complex, a UNESCO World Heritage Site, which also has the passage graves at Knowth (on view by guided tour) and Dowth (not open to visitors)."