This was actually discovered 2 years ago by extensive Geophys type stuff by Conor Newman and his report is available from the Discovery Programme based here in Dublin.

http://www.discoveryprogramme.ie/Research_Area/Research_Past.htm

It is a rock cut trench about 2m wide and about the same deep. Around the inside and outside there are huge post holes that must have held tree trunks, each one about 2m apart.

The date is speculative because no excavation has been carried out yet. It encircles the highest part of the hill and goes around the Rath Of The Synods, the modern church and the Mound Of The Hostages.

It's big! Oh yes it's very big! 150m long by 100m wide.

No sign of an entrance has been found and it is assumed to lie on the east side under the church yard. If you have the right light you can just see a slight depression in the field to the west of the Synods that shows part of its path, but this is only a couple of inches deep.

This enclosure is believed to be the oldest monument on the hill, which to me suggests a date pre 3000BC when the Mound Of The Hostages dates from (the oldest visible monument).

I'll send you a copy of some of the geophys if you'd like.