You may already know that you can't take photos inside the chamber unless you get prior permission from officialdom (and have substantial public liability insurance), but morning time is the best for outside pics since it faces south east. Look out for the barely noticeable carvings on some of the kerbstones going anti-clockwise from the entrance.

As Fourwinds says, definitely do Fourknocks and get the key up the road from Mrs. White (you need a 20 euro deposit). Loughcrew is only another 3/4 hour from Newgrange and if the coffee shop is open you can get the key for Cairn T, a must.

As for atmosphere, Newgrange really only comes to life at dawn and dusk, during the day the overwhelming quartz wall and gangs of tourists kind of sap away the vibes.

Here's some pics to whet your appetite: http://www.cianmcliam.smugmug.com/gallery/3899698/1/226224431/Large