The Down Survey Barony map of Clonmahon, created in the 1650s, depicts a substantial three-storey building with an attic at this location, providing one of the few visual records of what once stood here. This detailed survey, commissioned by Oliver Cromwell to redistribute Irish lands after the Confederate Wars, captured the castle when it was still standing; a rare glimpse into mid-17th century Cavan.

By the time the Ordnance Survey mapped the area in the 1830s, the castle had already disappeared from the landscape. The 1836 edition optimistically marked it as ‘Castle’ in italic lettering, suggesting some ruins might have still been visible, but by the 1876 edition, cartographers had resigned themselves to marking it simply as ‘Site of’. This gradual acknowledgement of the building’s complete disappearance reflects a pattern common across Ireland, where many castles and tower houses fell into ruin during the upheavals of the 17th and 18th centuries.

Archaeological surveys conducted in 1946 and 1948 found no visible remains at ground level, confirming that whatever stone once formed Lismore Castle had likely been robbed out for building materials over the centuries. The castle’s location within Lismore Demesne suggests it may have served as the principal residence for local landowners before being abandoned or destroyed. Today, visitors to the site will find only fields where this once-impressive structure stood, its existence preserved solely in the careful annotations of historical maps and the dry prose of archaeological inventories.