Donell Reogh O’Brien, who died in early 1600, made this tower house his residence, and historical records tell us he built it with the help of his younger brother Many O’Brien of Cornamagh. The castle appears on the 1837 Ordnance Survey map as a rectangular building running east to west, with field boundaries extending from its southeastern and southwestern corners, but by the time the revised maps were drawn in 1876 and 1910, no trace of the structure remained on paper.

Today, the site tells a rather different story from its days as an O’Brien stronghold. When archaeologists visited in 1979, they found absolutely nothing visible of the original structure or its enclosure; only modern field fences marked the boundaries where the castle once commanded the landscape. A dry stone wall now forms the western boundary of the field where the castle stood, whilst a school has been built in the neighbouring field to the west, bringing everyday life to what was once a seat of Gaelic power.

The complete disappearance of Coosan Castle sometime between 1837 and 1876 speaks to the broader fate of many Irish tower houses, which fell into ruin as political and social structures shifted in the centuries following their construction. These fortified residences, once symbols of local authority and family prestige, gradually gave way to the agricultural landscape that characterises much of rural Ireland today, leaving only historical records and the occasional field name to hint at their former presence.