Translating to 'the Boyne Palace', the Newgrange, Knowth and Dowth Neoliltic passage tomb complex is one of the most amazing ancient sites in Europe. Built in around 3200BC, it predates the Egyptian pyramids by around six hundred years. At one time a grain storage ("new granary") and at another time quarried, it is surrounded by Celtic Arthurian myth.
It had an unknown purpose (burial place for kings?), but I do know that it has mystical calendar qualities. I would absolutely love to be there when the sun lights up the 19m passageway lined with 43 upright stones on the December solstice to illuminate the chamber for 17 minutes. Lottery places only!
It's also amazing because the complex drainage system means no water has gotten in in it's centuries of existence. I found the carved kerbstones extremely beautiful -- triple spirals and other patterns.
It really is a massive mound -- 100m diameter. 97 boulders of the kerb ring stop the mound from collapsing, with 12 decorated kerbstones remaining.
Source: Lonely Planet Ireland 7th Edition