Built between 1219 and 1232, this fortress emerged during a turbulent period of Anglo-Norman expansion into Ireland. The castle’s history begins with John Marshall, possibly the brother of the famous William Marshall, Earl of Pembroke, who initially held the lands but hadn’t fortified them by 1219. By 1237, however, a stone house stood here, taken as security by the King against Marshall’s mounting debts. Over the following centuries, the castle passed through numerous hands; from Nicholas Dunheued in the 1270s to the le Bottiller and de Burgo families, eventually ending up with James Butler of Grenan in 1589.

The castle’s design follows the distinctive Marshall plan, also seen at Carlow, Lea, and Ferns castles. The structure consists of a central rectangular block measuring roughly 10.7 by 18.1 metres, divided by an internal spine wall into two chambers and fortified with four circular corner towers. The main entrance, elevated above ground level on the north wall, led into a chamber that gave access to the northwest tower containing a spiral staircase. The northeast tower, curiously inaccessible from the ground floor, may have served as a prison. The southern towers, reached from the south chamber, featured narrow arrowloops set into deep embrasures; classic defensive features of the period. A curtain wall once enclosed three sides of the castle, leaving the south side exposed, though only traces of this defensive perimeter remain visible today.

What survives today stands just one storey high, built from roughly coursed limestone rubble with the characteristic base batter typical of 13th-century military architecture. The walls still show evidence of internal rendering, suggesting the castle was once a complete, functioning fortress. Whether it originally stood two storeys tall or was always a single-storey structure with low angle towers remains a point of scholarly debate. A 1333 description mentions towers just twelve feet high, whilst architectural evidence and an 1833 account noting traces of an upper floor suggest it may have been taller. Despite its ruined state, Terryglass Castle remains an important example of early Anglo-Norman fortification in Ireland, its weathered stones bearing witness to over 800 years of Irish history.