A startling 80m in diameter and 13m high, Newgrange's white round stone walls, topped by a grass dome, look eerily futuristic. Underneath lies the finest Stone Age passage tomb in Ireland – one of the most remarkable prehistoric sites in Europe. Dating from around 3200 BC, it predates Egypt's pyramids by some six centuries.

The tomb's precise alignment with the sun at the time of the winter solstice suggests it was also designed to act as a calendar.

No one is quite sure of its original purpose, however – the most common theories are that it was a burial place for kings or a centre for ritual.

Newgrange's name derives from 'New Granary' (the tomb did in fact serve as a repository for wheat and grain at one stage), although a more popular belief is that it comes from the Irish for 'Cave of Gráinne', a reference to a popular Celtic myth. The Pursuit of Diarmuid and Gráinne tells of the illicit love between the woman betrothed to Fionn McCumhaill (or Finn McCool), leader of the Fianna, and Diarmuid, one of his most trusted lieutenants. When Diarmuid was fatally wounded, his body was brought to Newgrange by the god Aengus in a vain attempt to save him, and the despairing Gráinne followed him into the cave, where she remained long after he died. This suspiciously Arthurian tale (substitute Lancelot and Guinevere for Diarmuid and Gráinne) is undoubtedly a myth, but it's still a pretty good story. Newgrange also plays another role in Celtic mythology as the site where the hero Cúchulainn was conceived.

Over time, Newgrange, like Dowth and Knowth, deteriorated and at one stage was even used as a quarry. The site was extensively restored in 1962 and again in 1975.

A superbly carved kerbstone with double and triple spirals guards the tomb's main entrance, but the area has been reconstructed so that visitors don't have to clamber in over it. Above the entrance is a slit, or roof-box, which lets light in. Another beautifully decorated kerbstone stands at the exact opposite side of the mound. Some experts say that a ring of standing stones encircled the mound, forming a great circle about 100m in diameter, but only 12 of these stones remain, with traces of others below ground level.

Holding the whole structure together are the 97 boulders of the kerb ring, designed to stop the mound from collapsing outwards. Eleven of these are decorated with motifs similar to those on the main entrance stone, although only three have extensive carvings.

The white quartzite that encases the tomb was originally obtained from Wicklow, 70km south – in an age before horse and wheel, it was transported by sea and then up the River Boyne. More than 200,000 tonnes of earth and stone also went into the mound.

You can walk down the narrow 19m passage, lined with 43 stone uprights (some of them engraved), which leads into the tomb chamber about one third of the way into the colossal mound. The chamber has three recesses, and in these are large basin stones that held cremated human bones. As well as the remains, the basins would have held funeral offerings of beads and pendants, but these were stolen long before the archaeologists arrived.

Above, the massive stones support a 6m-high corbel-vaulted roof. A complex drainage system means that not a drop of water has penetrated the interior in 40 centuries.