From http://www.newgrange.com/index.htm:
“Newgrange is best known for the illumination of its passage and chamber by the winter solstice sun. Above the entrance to the passage at Newgrange there is a opening called a roof-box. This baffling orifice held a great surprise for those who unearthed it. Its purpose is to allow sunlight to penetrate the chamber on the shortest days of the year, around December 21, the winter solstice.

At dawn, from December 19th to 23rd, a narrow beam of light penetrates the roof-box and reaches the floor of the chamber, gradually extending to the rear of the passage. As the sun rises higher, the beam widens within the chamber so that the whole room becomes dramatically illuminated. This event lasts for 17 minutes, beginning around 9am.”

Newgrange recorded the 2007 event: http://www.newgrange.com/webcast.htm