The castle once stood at the northeastern corner of a rectangular bawn, commanding views across the valley to the east whilst the land rose behind it towards the summit of the hill. Today, only fragments remain of what was once a four-storey limestone tower house; the southwest angle is still visible, with foundation courses and short sections of the south and west walls surviving, the latter reaching a maximum height of just 0.83 metres. The grass-covered ruins measure approximately 6.4 metres north to south and 9 metres east to west, with evidence of small-scale quarrying visible throughout the vicinity, particularly on the steep southeastern face where the outcrop appears to have been deliberately worked.

The castle's history can be traced back to at least 1551, when Walter Butler, son of Thomas Butler of Mullaghmiony, served as a juror according to the Ormond Deeds. By the mid-17th century, the property belonged to John Butler of Mullaghneony, described in records as an "Irish Papist gentleman", whose lands included not just the castle but also two houses within the bawn, an orchard, and several cabins. The Ordnance Survey Letters from the 1830s provide a detailed snapshot of the castle before its complete collapse, describing a structure built of limestone with lime and sand mortar, featuring exterior render and a vaulted ground floor. At that time, the west wall still stood nearly 15 metres high, with visible joist holes indicating the four-storey layout, and remnants of plastering blackened with smoke on the underside of a stone arch.

The dramatic setting of Mullaghnoney Castle, positioned on its limestone perch that measures just 15.5 metres from south to north, speaks to the defensive considerations of its builders. Whilst time and quarrying have reduced this once-proud fortress to scattered stones and a displaced limestone door jamb at the base of the outcrop, the site continues to offer glimpses into the lives of the Butler family who once called it home. The rough cast exterior noted by 19th-century observers and the "appearance of modernism" they recorded suggest the castle underwent modifications over its centuries of use, adapting to changing times before ultimately succumbing to abandonment and decay.