This fortification of the O’Dempseys appears on several historical maps, including the 1563 map of Leix and Offaly and the Down Survey of 1655-6, yet its precise whereabouts remain disputed. The Ordnance Survey maps show conflicting locations; the 1906 edition placed ‘Clanmalier Castle (site of)’ west of Glenmalyre House, but according to 1959 OS Name Books, this was incorrect, and the actual site lay a few chains east of the house. Even the architectural historian Peter Harbison, examining Reverend Seymour’s eighteenth-century drawing of the castle, couldn’t pinpoint its location with certainty, noting that by 1839 no castle was marked in the townland of Ballybrittas at all.

What we do know about the castle comes largely from Reverend Seymour’s invaluable illustration, which shows a single upstanding gable with partial sidewall returns rising three storeys high. The drawing reveals fireplaces at each floor level, their flues feeding into distinctive double diamond-shaped chimney stacks; a feature that suggests construction around 1600. By 1794, the castle ruins stood on the right side of the road from Monasterevan, opposite what is now Rathdaire House, though by the nineteenth century the structure had seemingly vanished completely.

Recent discoveries have brought new life to this lost castle’s story. When the current owners of Glenmalyre House cleared soil from around their basement, they uncovered architectural fragments that almost certainly came from the medieval castle. These include limestone mullions from unglazed twin-light windows, one featuring a chequerboard pattern typical of the late sixteenth or early seventeenth century, and a chamfered jambstone complete with a strap hole for securing metal ties. A fourth fragment, now incorporated into a nineteenth-century bell tower, appears to be sandstone cornicing from a seventeenth-century fortified house. The difference in materials; limestone for some pieces, sandstone for others; hints at either two separate buildings or two construction phases, possibly a tower house that was later expanded with an attached fortified house.