Brú na Bóinne: Although these clearly served as burial mounds, some of them like Newgrange obviously had other ceremonial uses. Above the entry door was found a shaft which admits a beam of light to illuminate the central chamber only at dawn on the winter solstice (if the weather happens to be clear--no sure thing!). Because of the precession of the equinoxes, the illumination is a little off in modern times, but the alignment is still quite close. The restoration of the facade around the entrance is controversial, having been created by archeologists making guesses based on rubble in the 1960s. It's striking, but seems implausible. Visitors are taken inside the central chamber, which is awe-inspiring (photography forbidden).