It is roughly U-shaped in plan, open to the west, with three walls measuring between nine and nearly fourteen metres in length. The double-faced walls, built without mortar in the manner of traditional drystone construction, survive to a height of around 2.2 metres at their best-preserved southern section. Whatever once occupied the interior is now impossible to determine; collapsed stone has buried any trace of it entirely. The structure is classified as a folly, a category of building that was built for effect or whimsy rather than practical function, and which appeared frequently on Irish estates from the eighteenth century onward.

Knockmaa is a hill already layered with older significance. A prehistoric cairn, a large mound of stones typically marking a burial, sits some 285 metres to the east-south-east of the tower, and that cairn is itself crowned by a second folly. The proximity of these two constructed follies to a monument of genuine prehistoric weight gives the hillside an unusual character; someone clearly found Knockmaa a compelling stage for architectural statement, even if the precise motivations and the names behind the project have not been recorded. Whether the follies were intended to complement the ancient cairn visually, or simply to take advantage of the elevated ground, is not known.

The structure is partially collapsed and the interior offers no clear ground plan to read, so what a visitor encounters is more atmospheric than legible. The western opening frames the slope descending away from the hill, which may well have been a deliberate compositional choice by whoever commissioned the build.