It appears to be a modern insertion, which raises its own quiet questions: why put it there, and what, exactly, is it mimicking? The building itself is a rectangular structure measuring roughly 6.63 metres by 11 metres, with walls 0.6 metres thick, aligned northwest to southeast and sitting just west of the monastic complex at Holycross. It has been heavily repointed and repaired over the years, and most of its windows are modern rebuilds with later surrounds and lintels, giving it the slightly ambiguous quality of a building that is old in outline but extensively renewed in detail.

What makes the mill's working mechanism immediately legible is the millrace, a channel cut to divert river water, which runs parallel to the Suir before passing the southeast gable. From that gable, a wooden undershot mill-wheel, meaning one driven by water striking its lower blades rather than flowing over the top, projects outward through an opening of roughly 0.6 metres square. The Civil Survey of 1654 to 1656 records that the lands of Holycross supported two or three grinding mills and a tucking mill, the latter being a fulling mill used to clean and thicken woven cloth by pounding it in water. Whether this surviving structure is continuous with any of those recorded mills is not certain, but the documentary evidence establishes that milling was already a significant industrial activity on these lands in the mid-seventeenth century, almost certainly sustained by the same river that still runs alongside the building today.