It is a relic of a Cistercian abbey, Inishlounaght, whose stonework was quarried and reused when the courthouse was raised in the late seventeenth century. That a fragment of medieval monastic fabric, the kind of mark a bishop would have made at a church's dedication, survived the demolition, the transport, and the rebuilding is quietly remarkable. It is not the only recycled piece; traces of fluting are still visible at the base of a southern column in the arcade, another remnant of the abbey worked into a building of an entirely different ambition.

The Main Guard was commissioned by James Butler, first Duke of Ormond, and constructed between 1673 and 1684 as the courthouse for the Palatinate of County Tipperary, a jurisdiction that gave the Ormond earls and dukes quasi-royal legal authority over the county. The building's architect was possibly Sir William Robinson, who served as Surveyor General of Ireland, and the design bears the clear influence of Sir Christopher Wren, whose London work was reshaping architectural taste across the British Isles at the time. The structure is long and rectangular, built in sandstone ashlar, with an open arcade at ground-floor level, a Mansard roof, a heavy cornice, and an octagonal lantern that may be original. Two heraldic sandstone plaques, dated 1675, sit on the western facade at first-floor level; one carries the eroded arms of Butler of Ormond, the other the arms of the Borough of Clonmel. The palatinate jurisdiction itself was extinguished around 1715, after which the building served as a tholsel, a combined town hall and commercial exchange, and then as a seat of the Assize Courts until a new courthouse was completed in 1803. Around 1810, the ground-floor loggia of open arches was converted into shops, a pragmatic alteration that obscured much of the original character until the building was taken into State care in the 1990s and restored closer to its seventeenth-century appearance.

The arcade is worth examining slowly. Several of the half-round engaged columns on the interior still carry their original capitals, and the easternmost pier on Mitchell Street, where that consecration cross survives in its plaster, rewards a close look. The cross would have been cut into the wall of the abbey church at Inishlounaght as part of its formal consecration, and its presence here, folded into a ducal courthouse without apparent ceremony, says something odd and interesting about how building materials moved through the post-Reformation landscape of Munster.