In 1842 the Ordnance Survey recorded it clearly, using hachures, the short radiating lines cartographers use to indicate raised ground, to mark a circular mound. By 1904 it had disappeared from the map entirely, and by 1937 it reappeared, but this time rendered not as a monument at all, just as a subrectangular field boundary roughly ten metres across. The monument itself had not moved, of course. What changed was how surveyors chose, or failed, to see it.

What survives on the ground is a ring barrow, a burial monument of prehistoric date in which a low circular mound is defined by a surrounding ditch, known as a fosse, with a low bank thrown up on its outer edge. At Knocknanagh the enclosed area measures roughly 7.5 metres north to south and 9.2 metres east to west, with the external bank still standing to a height of around half a metre. That is modest but legible in the landscape, particularly on a slope where earthworks tend to survive better than on heavily worked arable ground. There is some disturbance to the interior on the western side, which is not unusual for monuments that have spent centuries at the edge of farmland, occasionally caught by a plough or used as a convenient marker for a field division.