That discrepancy is, in its quiet way, the most interesting thing about this enclosure. When the Ordnance Survey first mapped it in 1839, they recorded a square outline roughly 38 metres on each side. When surveyors returned for the 1899 to 1900 revision, the same feature had become sub-circular, with a diameter of around 45 to 50 metres. Whether the original corners had always been an approximation, or whether the earthwork itself had softened and spread over sixty years of agricultural life, the two maps together raise a question that neither fully answers.

Enclosures of this kind are common enough across the Irish midlands and south-east, often the remains of a rath or ringfort, the circular or oval earthen enclosures that served as farmsteads and defended homesteads throughout the early medieval period. What gives Tullamaine a slightly different character is the water surrounding it. The 1839 map shows an adjoining pond on the eastern side, roughly 20 metres at its longest, with three further natural ponds clustered 10 to 20 metres away to the south-east, south, and south-west. That concentration of water on multiple sides would have made the enclosure considerably more defensible, or at least more secluded, than its earthworks alone might suggest. A field boundary running east to west, about 5 metres north of the northern perimeter, hints at the layers of agricultural reorganisation that have accumulated around it over the intervening centuries.