The site occupies a low rise on a high bluff overlooking a bend in the River Dunneill, offering commanding views of the surrounding countryside. Though no masonry remains visible today, this subcircular mound, measuring roughly 30 metres northeast to southwest and 25 metres northwest to southeast, tells a story of centuries of Irish clan politics and conflict.

The castle took its name from Neill, a chieftain of the O’Dowd family who originally held this strategic position. The O’Dowds were one of the prominent Gaelic families of north Connacht, but like many Irish strongholds, Dunneill Castle changed hands over the centuries, eventually falling under the control of the MacSweeneys. Historical records place the castle firmly in local history; it appears as ‘Dooniell Castle’ on the Down Survey maps of 1655-6, one of the most comprehensive land surveys of 17th century Ireland, whilst the 1837 Ordnance Survey map marks it simply as ‘Site of Dunneill Castle’, suggesting the structure was already in ruins by that time.

The castle’s final chapter came during the turbulent 1640s. According to depositions from 1641, during the early days of the Irish Rebellion, the castle was under the charge of one ‘Gullne duff O’Hart of Carbury’. Today, the mound shows considerable evidence of past quarrying and disturbance, its height varying from 1.7 metres on the northwest side to 3 metres on the southeast. While the stone and mortar have long since been carried away, the earthwork remains as a tangible link to the Gaelic lordships that once controlled this part of Sligo.