Built on a T-plan typical of 18th-century Irish country houses, the building presents a fascinating archaeological puzzle: was it constructed on the site of an earlier castle, or does it perhaps incorporate remnants of one within its walls? The question arose from two tantalising references by historian Neary in 1913-14, who mentioned that ‘Knockglass Castle has also disappeared from the scene to make room for a modern residence’, listing it amongst Mayo’s lost castles as ‘now razed’. Yet curiously, no castle appears on any Ordnance Survey maps, nor was one mentioned in the detailed OS Letters of 1838, leaving the source of Neary’s information frustratingly unclear.

In 2009, archaeologist Timoney conducted a pre-development assessment of the property ahead of conservation works, searching for evidence of this elusive castle. His investigation uncovered some intriguing architectural features that hint at an older structure; particularly notable are two unusually thick internal walls and several low arched doorways in the rear section of the house, which he suggested might incorporate elements of an earlier fortification. However, the evidence remains circumstantial at best, with no dressed stone, base batter, or other definitive castle features found either in the main house or amongst the outbuildings.

Today, the National Inventory of Architectural Heritage recognises Knockglass House as a fine Georgian country house, complete with its original suite of farm buildings, walled garden, and gate lodge, but makes no mention of any medieval predecessor. Whether the house truly stands on ancient foundations or Neary’s castle was simply a case of mistaken identity remains an open question, adding an extra layer of intrigue to this already impressive piece of Mayo’s architectural heritage. The mystery of Knockglass Castle serves as a reminder that Ireland’s historical landscape often holds secrets that even careful archaeological investigation cannot always resolve.