The two passages are closed. One has a gate that you can see in through, the other is below ground level and you can see nothing of it. The latter one leads to two chambers and has masses of carving in it and a beautiful basin in the centre of one of the chamber.

Once upon a time the lady that owned the field used to take people down there with a candle! Wow! How amazing would that have been? All those carvings dancing in candlelight.

Access to the mound is itself is open. You can walk around the mound and check out the warn carvings on the outside. I've never been sure if there's some sort of conspiracy by the visitor centre to stop people just going to Dowth (which is by far the best of the three sites, because of its rawness) or the staff there simply don't know what is at Dowth. Many of them have probably never been there.