What survives, despite considerable collapse, is enough to read the building's interior logic: a two-storey limestone rubble structure, roughly nineteen metres by ten and a half, divided at ground level by a now-vanished cross-wall that separated a smaller southern room from a larger main space. A slop-stone, a shallow drainage feature set into the wall for disposing of waste water, still sits in the larger ground-floor section near where that partition once stood. Upstairs, ogee-headed windows, their curved and pointed arch profiles typical of late medieval Irish domestic architecture, once lit both chambers, though most of what remains of them is fragmentary. There are no fireplaces, no chimneys, and no garderobe, which is notable for a house of this size and pretension.

The castle's history is bound up with a branch of the Butler family, one of the great dynasties of medieval Munster. Theobald Butler of Roosca served as Sheriff of the Liberty of Tipperary in 1591 and 1592, and when he died in 1595 or 1596 his son Pierce inherited. Pierce's tenure was brief and unhappy: he was attainted, meaning his lands were legally forfeited as a consequence of political or legal offence, and by 1614 they had been granted to a Henry Piers of London. William Butler, an uncle of Pierce, subsequently recovered the title from Piers and died at Roosca in 1634, leaving the property to his son Theobald. It is this later Theobald who appears in the Civil Survey of 1654 to 1656, which records the castle as it stood in 1640, describing it as "an old stone house coverred with thatch within a bawne." A bawn is an enclosing defensive wall, typically of stone, built around a house or tower to protect its immediate ground. At Roosca the bawn still partly stands, running northeast from the house and north from its gable, with a roughly circular two-storey tower wedged into its northeast angle. That tower retains shot-holes at both levels, small openings angled to allow firearms to be directed outward, a reminder that even a modestly described thatched house needed to be defended.