On a rainy day I trudged across the field to this henge for a look. Rarely mentioned, at least in the books that I've read, I wasn't expecting much. Don't believe what you don't read! This is well worth the effort.
I can't begin to estimate the diameter or circumference, but can say with confidence that either of the Knowth or Newgrange mounds would fit inside it with plenty of room to spare. It's huge! The banks of the henge rise to about 5 metres. There are 2 distinct openings, roughly to the south-west and north-east, but there may be more – there's a lot of heavy growth at this time of the year. This henge is on private land so you may need to get permission.
I would have liked to stay here longer but the rain was rotten and I was dressed for summer. Definitely one that ought to be on your itinerary when you visit Brú na Boinne. I'll certainly be back.