The ground floor of this institution incorporates the remains of a 16th-century castle, built in 1561 by John Bathe, who would later serve as Chancellor of the Exchequer. A commemorative tablet from that year, preserved on an internal wall on the north side of the building, marks this historic foundation. The Bathe family of Meath constructed their castle as a substantial rectangular structure, possibly featuring a turret on its southwest corner, with walls nearly 1.2 metres thick that still stand today.

The castle’s original features remain surprisingly intact at ground level, where they now form part of the asylum’s kitchen. Visitors familiar with medieval architecture would recognise the telltale signs: the exceptional thickness of the walls, the surviving vaulted ceilings, and a rectangular fireplace embedded in the southern wall. After John Bathe’s death in 1586, the castle passed to Sir William Warren through his marriage to Bathe’s widow, Eleanor Preston. Historical maps from the Down Survey show it as a substantial castle house flanked by two smaller gabled buildings, whilst records from 1703 describe an estate that included not just the castle but also a brick dwelling house, stables, coach house, malt house, and various outbuildings.

Archaeological investigations in 2009 revealed further traces of the castle’s past, with test excavations northwest of the site uncovering a series of pits and ditches believed to be connected to the original medieval structure. This layering of history, where a Victorian asylum stands atop and incorporates a Tudor castle, creates one of Dublin’s more unusual architectural palimpsests; a building where centuries of Irish history quite literally support one another.