The Bru na Boinne centre is very good. It is built into the hillside on the banks of the Boyne. You can see Newgrange through one of those static telescopes from a viewing gantry. The roof is turfed and planted so well that you really can't tell it's there even from across the river. It's impossible to spot from Newgrange.

There is a (not too bad) audio visual room and some good displays. There's also a model of one of the passages split down the middle so you can see into it. At the Stonehenge one you could erect foamhenge in the carpark :-) as long as it was well out of sight from the real thing it'd be fine.