Built by the O’Dowds, who ruled as Lords of Tireragh, the rectangular fortress once stretched approximately 32 metres northwest to southeast and 21 metres east to west. Today, only fragments remain: the southeastern wall stands 8 to 10 metres high, whilst just a corner portion of the northwestern wall survives. The surviving architecture showcases sophisticated medieval defensive features, including machicolated projections supported by hefty brackets, complete with gun loops, and walls with a slight batter that rest on a supportive plinth buttress.

The construction reveals skilled craftsmanship, with precisely cut quoins at the corners and an ashlar finish concealing the random rubble and lime mortar core. The 1.35 metre thick walls feature architectural details like a neatly angular corner chamfer on the southwestern end and a central window with an ashlar surround. The northeastern section, where the ground naturally rises, shows sod covered remains that might indicate a bastion type entrance to the castle. The surrounding landscape tells a broader story of medieval settlement, with earthworks to the west, a house site to the south, and a moated site to the southwest.

Ardnaglass Castle once housed a remarkable stone sculpture that now resides in the National Museum of Ireland. This bas relief carving depicts two intertwined animals, traditionally interpreted as a dog killing a wolf, which was documented by antiquarians Wood Martin in 1882 and Kilgannon in 1926. Though the castle appears as ‘Ardnaglass Castle’ on the 1913 Ordnance Survey map, its walls have witnessed centuries of Irish history, from the medieval power of the O’Dowds through to its current state as an evocative ruin in the Sligo countryside.