The castle appeared on the Down Survey map of 1655 to 1656, sitting roughly where Neillstown House would later be built, just north of what’s now the 9th lock on the Grand Canal. By the time the Civil Survey documented the area between 1654 and 1656, it was already described as ‘the ruins of an old castle’, suggesting the structure had been abandoned or destroyed sometime before the mid-17th century.

The exact history of the castle remains something of a mystery, as is often the case with smaller fortifications that didn’t belong to particularly prominent families or play roles in major historical events. What we do know is that by the 1650s, when Oliver Cromwell’s surveyors were mapping Ireland in meticulous detail, Neillstown Castle was already a shadow of its former self.

Today, the site has been completely built over, leaving no visible remains at ground level. It’s one of countless small castles and tower houses that once dotted the Irish landscape; structures that served as both homes and defensive positions for minor nobility and landowners. While grander castles have been preserved or at least left as picturesque ruins, Neillstown Castle has vanished entirely beneath centuries of development, remembered only in old maps and survey records.