The cairn at Knockeen is not a dramatic monument by the standards of Irish prehistory, but its position is quietly telling. Cairns of this kind are essentially piles of stone, sometimes covering burials, sometimes marking territory or memory, their original purpose often irretrievable. This one sits near the middle of Knockeen hillfort, a placement that suggests it was either respected by the fort's builders or, quite possibly, the reason the enclosure was built around it in the first place.

The mound is subcircular in plan, meaning roughly oval rather than a true circle, measuring about 18.7 metres from north to south and 10 metres from east to west, with a height ranging between 1.8 and 2.5 metres. It has been covered over with earth, which is why it reads more as a grassy swelling in the landscape than a stone heap. At some point, field stones were added to the eastern and western sides, a detail that points to later interference, perhaps by farmers clearing nearby ground and finding the mound a convenient place to deposit loose stone. That kind of incremental alteration is extremely common with ancient monuments in agricultural landscapes, and it makes interpreting the original structure considerably harder.