Inside the walls, which are a metre thick and roughly 5.5 metres across, the stonework steps back at two distinct levels to form narrow ledges, the kind of ledge that once supported timber floors. Doorways face each other on the north-east and south-west sides, a pairing that would have allowed a miller to move freely between the machinery on different levels. This is what remains of a windmill, a building type that was once far more common in the Irish midlands than the surviving examples suggest.

Windmills in Ireland were most often built from the seventeenth century onwards, when improving agricultural output and the milling of grain became pressing economic concerns for landlords and farming communities alike. The Cannorstown tower is thought to date from either that period or the following century, with one assessment placing its construction at around 1800. It is built of coursed rubble limestone, a practical and locally available material throughout County Westmeath, and its circular plan, standard for tower windmills, was designed to allow the cap and sails to be rotated to face into the prevailing wind. By the time the structure was recorded in 1980 part of the upper wall at the second floor level had already collapsed on the north-east side, and the building has been out of use and roofless for a long time. What makes it worth noticing is its position: sited close to the summit of a gently sloping hill, it sits in a spot with wide views in all directions, which was precisely the point. A windmill needed exposure, and whoever chose this location understood that well.