Not demolished in the conventional sense, not burned or quarried away in living memory, but simply absorbed, until no trace of it remains visible at ground level. That erasure alone makes the site quietly remarkable.

The historian William Carrigan, writing in 1905, noted the presence beside Killahy Castle of what he described as the extensive ruins of a fine mansion dating to around the year 1700. Ordnance Survey mapping adds some texture to the story. The first edition six-inch OS map, surveyed in 1839, shows at least one building in the area as still roofed; by the 1900 revision, it appears as ruinous. The later twenty-five-inch OS map records only the bare skeleton of one possible candidate, a southeast wall running to about fourteen metres, with northeast and southwest gables roughly four metres wide and a roofless square chamber at the southwest end. A second building, considerably larger at approximately twenty-six metres by six metres and located about forty metres to the northwest of the castle, was also roofed in 1839 and in ruins by 1900, and may equally be what Carrigan had in mind. The uncertainty is genuine; the cartographic and historical evidence points to the general area without settling the question of which structure, if either, was the mansion in question. What is clear is that between the mid-nineteenth century and the early twentieth, something sizeable was lost, and that whatever remained above ground has since been swallowed entirely by grass and soil.