This roughly circular platform, measuring about 20 metres across, rises dramatically from the surrounding pasture with steep sides reaching 2.3 metres in height. The summit, marked at 503 feet on old Ordnance Survey maps, commands sweeping views across the countryside in every direction. At its centre, a low mound of earth and stones now supports an Ordnance Survey triangulation station, whilst the eastern and southern portions of the scarp show evidence of past quarrying that has left a distinctive concave indent in the monument’s perimeter.

The true nature of this hilltop earthwork remains tantalisingly uncertain. Some scholars, including Adams writing in 1959, have suggested it might be a motte; one of those earth and timber castles built by the Anglo-Normans following their arrival in Ireland in the 12th century. The flat-topped platform certainly fits this interpretation, and such fortifications were common throughout the medieval period. However, the monument’s prominent hilltop position and its form could equally point to much older origins as a prehistoric burial mound, perhaps dating back thousands of years to the Bronze Age when such elevated sites were favoured for important burials.

Adding to the mystery, some researchers have proposed it might be a medieval raised rath or ringfort; essentially an enhanced version of the circular earthen enclosures that dot the Irish landscape. Without excavation, the monument keeps its secrets well. What is certain is that this imposing earthwork, whether ancient tomb, medieval fortress, or something else entirely, has dominated the summit of Sion Hill for centuries, serving various purposes from possible stronghold to modern survey point. Another enclosure lies just 62 metres to the east-southeast, hinting at a landscape rich with archaeological remains waiting to tell their stories.