The castle was most likely constructed in the 15th century by the Bishop of Ferns as a summer residence, following the settlement of the manor on the bishopric between 1228 and 1232 by the monks of Christchurch, Canterbury. The building’s unique design features two separate households, possibly allowing the bishop to share the residence with the rector. Bishop Thomas Denne, who served from 1363 to 1400, may have commissioned the castle as he sought to relocate the diocesan centre away from Ferns during the Gaelic resurgence. Throughout the 16th century, the bishops continued to reside at Fethard, though the property faced numerous challenges including an attack in 1532 by Cahair McArt of Macmurroughs Island, who plundered twenty inhabitants with the surprising assistance of Bishop John Purcell, and returned two years later to drive off 115 cattle.

The castle’s complex L-shaped structure reveals its sophisticated medieval design, built as a unified structure with a circular tower at the southwest corner. The main east-west block incorporates an earlier gatehouse at its eastern end, whilst a wing extends northward from the western end. What makes this castle particularly fascinating is its three distinct, originally inaccessible units: the vaulted ground floor accessed through a doorway on the south side, the first and second floors of the east wing, and the north wing. The gatehouse passage, complete with hinge-stones for a drawbridge, leads to various chambers including a magnificent first floor hall measuring 12 metres by 4.7 metres. Notable features include a granite fireplace in the north wall, cusped ogee-headed windows in Old Red Sandstone and Dundry stone, and rather grimly, an oubliette or hidden chamber known as ‘God’s Black Pit’, where Bishop John Purcell allegedly imprisoned Dean Thomas Hay in 1569 to seize his Papal Bull appointment.

Following the Reformation, the castle’s ownership shifted dramatically; Alexander Devereux, the last abbot of Dunbrody who became bishop from 1539 to 1566, along with his nephew and successor John, alienated much church property to relatives. Protestant bishop Thomas Ram recovered Fethard between 1605 and 1634 before exchanging it with Sir Nicholas Loftus for other properties. During the 1641 rebellion, Catholic forces under Captain James Downes occupied the castle, but it eventually returned to the Loftus family who retained it into the 20th century, making substantial alterations during the 18th and 19th centuries. Now under the ownership of Wexford County Council, the castle is undergoing conservation work directed by B. Murtagh, though the interior remains completely inaccessible to visitors, preserving its mysteries behind ancient walls that have witnessed centuries of ecclesiastical politics, family feuds, and architectural evolution.