Built by Robert Hartpole, who received an extensive grant of lands in 1576 including ‘a ruinous castle and lands in Clonrere’, the tower measures nearly 12 metres east to west and just under 11 metres north to south. Its roughly coursed limestone walls survive up to wall-walk level, though the original parapet is long gone, replaced by modern coping on the south and west walls. A limestone plaque on the south wall bears an inscription with the letters RHK and the date 1640, whilst another inscription reading ‘1576 ROBART HARPOL GRAIN HARPOL DOLAGH O LALOR’ was once visible on the fireplace arch stone on the first floor.

The castle’s interior reveals centuries of modifications alongside its original medieval features. The ground floor, accessed through an inserted eastern doorway complete with murder hole above, has been divided by a cross-wall into two barrel-vaulted chambers that still retain plaster over their wicker centring. A spiral staircase in the southeast angle rises through all four floors, providing access to chambers that mix original elements like ogee-headed windows and squints with later insertions. The first floor contains a large, now-destroyed fireplace in the south wall, whilst the third floor retains corbels that once supported its ceiling and features a fireplace in the north wall. Throughout the tower, defensive features like drawbar sockets and iron pintles for door hinges remain in place, testament to its original military purpose.

By the 18th century, Shrule had evolved from fortress to family seat. Austin Cooper, visiting in 1782, described it as ‘a small, square modernized castle, with modern wings and additions’ and noted its beautiful situation about four miles from Carlow on the Queen’s County side of the Barrow. An 18th-century house was once attached to the north wall, though this has since been demolished, leaving only farm outbuildings attached to the west. Despite these various architectural alterations over the centuries, the castle remained habitable well into the modern period, serving as the ancient seat of the Hartpole family and standing as a remarkable example of how Ireland’s tower houses adapted to changing times whilst retaining their essential medieval character.