Newgrange

A passage tomb in County Meath built around 3100 BC, five hundred years older than Stonehenge and older than the Great Pyramid. Above the entrance is a slot called the roofbox, and it is not decoration: at the winter solstice the rising sun comes straight down the passage and lights the inner chamber for about seventeen minutes, falling on the triple spiral carved on the back wall. Whoever built it worked out the geometry, cut the shaft, and got it right, and it still works five thousand years later. To be inside for the real thing you enter an annual lottery; tens of thousands apply and sixty are drawn. The rest of the year you go by guided tour from the Brú na Bóinne visitor centre and they simulate it with electric light, which is honest and still worth doing. UNESCO listed the complex, with Knowth and Dowth, in 1993.