Pollnaveagh Well is a subterranean chamber, its shaft around two and a half metres in diameter and lined with mortared stone, tapering as it descends. A flight of shallow steps leads down from the east-southeast to a narrow pointed entrance, just sixty-five centimetres wide but over two metres tall, and above that opening sits something that catches the eye: a fragment of a medieval two-light ogee-headed window, the kind of decorative tracery more at home in a church or a great house than at the mouth of a fieldside well. Alongside it stands a plain, roughly hewn stone cross on a round base. Locally, the site has been known as St. Matthew's Well, which would place it in the long Irish tradition of holy wells, springs or sources associated with a particular saint and visited for devotional or curative purposes.

The ambiguity is what makes Pollnaveagh genuinely interesting. The well appears on the Ordnance Survey six-inch maps of both 1838 and 1922, so it has been a recognised feature of the landscape for at least two centuries. But its location sits squarely within the estate of the Mahon family of Northampton House, and the Mahons were enthusiastic builders of follies, ornamental structures designed more for visual effect or whimsy than for practical use. Paul Gosling, writing in 1987, pointed out that the medieval window fragment surmounting the entrance, the hewn cross, and the general theatrical quality of the descent suggest this could be another of the family's constructed curiosities, a fabricated antiquity dressed up to suggest age and sanctity. A small mortared recess near the base of the steps on the north side adds to the puzzle; its function was unclear even to those who examined it, and it had been disturbed before any record was made.