The castle’s origins date to 1408, when Patrick Barrett, Bishop of Ferns, received permission to employ masons for its construction. What remains today is the largest tower house in Wexford, measuring 13.8 metres north to south and 10.8 metres east to west. The core of Barrett’s original structure likely forms part of the current building, though Sir Richard Butler, who became the first Lord Mountgarrett in 1550, appears to have substantially rebuilt it into its present five-storey form.

The tower house originally rose to a wall walk, featuring defensive elements typical of its era: a pronounced base batter on the north wall, quoins at the corners, and a pointed entrance with a yett on the south wall. Inside, a guardroom once stood to the east of the entrance, whilst a spiral staircase occupied the southwest corner. The upper floors contained fireplaces and four garderobe chutes in various locations, alongside defensive features including musket loops at the northwest angle and a large mullion and transom window with gun loops on the second floor’s north wall. The castle was once protected by a bawn; a defensive wall measuring approximately 35 metres east to west and 60 metres north to south, traces of which remained visible as late as 1839.

Recent decades have not been kind to Mountgarrett Castle. Severe frosts in the winter of 2009-10 led to the collapse of the south wall and parts of the west wall in February 2010. Despite remedial works carried out in 2012, the northeast angle collapsed in early February 2016, leaving the structure in a precarious state. These collapses have transformed what was once a formidable defensive residence into a fragile monument, requiring careful preservation to maintain what remains of this significant piece of Wexford’s architectural heritage.