The tower house, measuring 11 metres east to west and 7.5 metres north to south, stands four storeys high despite its partially ruined state. Built from rubble limestone with dressed quoins, the castle’s walls show evidence of multiple building phases spanning from possibly the 13th century through to the 18th century. Medieval narrow windows were blocked up over time whilst tall brick-lined windows were inserted into the south wall, and the original pointed limestone doorway in the north wall, complete with grooves for an iron-barred gate, speaks to its defensive origins.

The castle’s documented history begins in 1551 when Sir Ralph Bagenal received a lease for lands including ‘a castle in decay’ at Coolbanagher, suggesting the structure was already ancient by Tudor times. By 1563, Robert Hartpole of Carlow acquired the property along with surrounding lands, holding them in exchange for maintaining six English horsemen for Crown service. His son, Sir William Hartpole, made Coolbanagher his residence in the late 16th century and likely commissioned significant alterations including the insertion of wall fireplaces and the distinctive pointed doorway. Knighted at St. Mary’s Abbey in 1603, Sir William transformed what had been a medieval stronghold into something more suited to the needs of an Elizabethan gentleman.

Today, the castle stands 35 metres west of a post-1700 house, with the medieval ruins of Coolbanagher church visible 90 metres to the southwest. The 1837 Ordnance Survey map shows it positioned along the boundary wall of a walled orchard, though only the northern wall of this enclosure survives. The castle’s fabric tells a complex story; the north, south and east walls remain standing to the string course at roof level, whilst the western wall has partially collapsed. Evidence of blocked windows, broken-out openings, and the possible crease line of a vanished building against the western wall all hint at the various lives this structure has lived, from medieval fortress to Tudor manor house to eventual abandonment.