interesting @Christian427:
I was there with a friend in 1995. We were even able to spend the night on the lawn next door. A great experience with a couple and in the morning after breakfast together we went on a tour of Newgrange. Today we were there: a mega visitor center, but with (entrance fee: 5 euros) it was quite OK and really well prepared. However, we were on our way back to Dublin and the next bus didn't leave for 1.5 hours later. It would not be possible to visit from the inside, only from the outside (according to the lady at the ticket office). The shuttle bus tour would be just over 50 minutes and with a stopover around 2 hours (but can only be booked online in advance). You don't have to/can't book the visitor center alone online.