I didn't cross the field to visit this site, but it is so obvious as you visit the nearby court tomb at Tanrego West (County Sligo) that I had to mention it. It is not marked on the OS maps - nor is the larger ringfort (see next site) and the standing stone (which I missed) in the same field.
The closeness of this and the other monument mentioned above to the court tomb could be significant. Where they built here to claim the power of the ancient stones?
This one is about 30m across and planted with dense trees.
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This is an explanation of (and a bit of a disclaimer for) the coordinates I provide.
Where a GPS figure is given this is the master for all other coordinates. According to my Garmin these are quite accurate.
Where there is no GPS figure the 6 figure grid reference is master for the others. This may not be very accurate as it could have come from the OS maps and could have been read by eye. Consequently, all other cordinates are going to have inaccuracies.
The calculation of Longitude and Latitude uses an algorithm that is not 100% accurate. The long/lat figures are used as a basis for calculating the UTM & ITM coordinates. Consequently, UTM & ITM coordinates are slightly out.
UTM is a global coordinate system - Universal Transverse Mercator - that is at the core of the GPS system.
ITM is the new coordinate system - Irish Transverse Mercator - that is more accurate and more GPS friendly than the Irish Grid Reference system. This will be used on the next generation of Irish OS maps.