This substantial medieval fortification consists of an oval, flat-topped motte standing nine metres high at its steepest point, perched atop a distinctive triangular bailey that stretches approximately 50 to 56 metres on each side. The motte itself, measuring 10 metres east to west and 8 metres north to south, still bears the remnants of a castle on its summit, along with what Seymour described in 1916 as 'an open hole in top, which probably led into a chamber'. Today, a modern statue of Jesus stands slightly off-centre to the west, an unexpected addition to this ancient defensive structure.

The site's history stretches back to the late 13th century when it served as the manor of 'Donethyl', held by the Le Ercedekne (or Archdeacon) family. In 1295, the castle became the scene of dramatic events when Silverster le Ercedekne lodged a complaint against Hugh Purcel, the Sheriff of Tipperary, accusing him of forcibly entering the castle, breaking open chests and coffers, and making off with goods worth £500; a staggering sum for the time. The account of this raid mentions a causeway several times, noting it was 'broken hastily', suggesting it was likely a wooden structure that either provided access to the castle on the motte or crossed the surrounding marsh. By 1319, the importance of Donoghyl manor was evident when it received permission to hold a weekly market with liberties and free customs, marking it as a significant economic centre in medieval Tipperary.

The engineering of the fortification reveals sophisticated medieval defensive planning, with the triangular bailey divided into three distinct sub-baileys at the eastern, southwestern and northwestern corners, connected by narrow passages just 3.8 metres wide. The eastern approach appears to have been the main access point to the motte's summit, where the scarp falls more gently. Aerial photography from 1968 reveals traces of an outer rectangular enclosure to the north, visible as cropmarks defining a fosse, possibly the faint remains of outworks that once extended into the marsh. The site sits within a broader historical landscape that includes a medieval church and graveyard just 110 metres to the east, and St. James' holy well 180 metres to the northeast, creating a complex of religious and defensive structures that dominated this part of Tipperary during the Middle Ages.