It's a shame you can't get into the chamber at Knowth. The artificial room that tour parties are led into might as well be in the Visitor Centre.
The view from the top really gives you a sense of the place of this tomb within the landscape and I could spend hours looking at the rock-art on the displayed kerb-stones.
The later use of the site by Celts and Normans are also of interest, in particular the creep passage east of the main mound. Children of all ages (!) can crawl through it during the guided tours.