Should You Take The Tour?
My answer is an emphatic YES! That's the only way you're allowed to see the inside of Newgrange. They are very protective of this place, and rightly so.
It was originally discovered a couple centuries ago (if I remember correctly), and was "vandalized" by old-school graffiti - people etching and carving their own names into the stones to be immortalized.
Also, if you take the tour, you'll be asked to take off any backpacks or purses you're wearing so they don't scratch the walls or ceiling.
If you're with a large group, they have you all stand in the middle, and do a quick re-enactment of how it would look on Winter Solstice, when the sun illuminates the passageway directly into the center of the tomb.