The bright white wall that greets visitors to Newgrange today is a twentieth century decision, not a certain reconstruction of the original.
What O'Kelly found and built
Excavating between 1962 and 1975, Michael J. O'Kelly found thousands of white quartz cobbles and rounded granite stones fallen at the base of the mound's front, mixed together on the ground. He rebuilt them as a near-vertical retaining wall of quartz studded with granite boulders around the entrance, the striking facade now shown in every photograph of the site. Other archaeologists have argued since that the quartz more likely formed a low spread or apron in front of the mound, since a freestanding dry quartz wall of that height would be structurally difficult to justify from the fallen evidence alone.
A tomb wrapped in later stories
Newgrange predates the pyramids of Giza and Stonehenge's sarsen circle, yet medieval Irish tradition folded it into much later mythology, naming it a residence of the Tuatha De Danann and associating it with the god Aengus, who is said to have won the mound by a trick of wording rather than force.
Rediscovery
The mound was reopened to record in 1699 after a landowner's workers, digging for stone, broke into the passage and chamber, exposing a monument that had been sealed for roughly four thousand years.