Locally it goes by the name Lyon's Fort, a designation recorded by Neary as far back as 1914, which suggests the family name had been attached to the site for long enough to stick in the townland's collective memory. The structure beneath that name is considerably older, belonging to a type of enclosure built predominantly in the early medieval period, roughly between the fifth and twelfth centuries, and used as a farmstead or place of status by a family of some local standing.

A rath, as this kind of ringfort is often called, typically consists of a raised circular platform enclosed by one or more earthen banks with a ditch, known as a fosse, dug between them. At Knockballyvishteal the overall diameter is approximately 52 metres, which places it solidly in the middle range for such monuments. What survives today is a scarp, an intervening fosse, and an outer bank, though the enclosing elements have been worn away almost entirely on the north-east side, leaving no visible surface trace. A gap on the south-west looks to be of modern origin rather than the original entrance, and a field bank, almost certainly the result of later agricultural boundary-making, cuts through the monument on both its northern and eastern sides. The combination of these intrusions, centuries of ploughing, grazing, and general land management, has left the site in poor condition, which perhaps explains why the local name, with its reference to the Lyons family, has outlasted any clear sense of what the earthwork actually is.