The Wonderful Barn is a corkscrew-shaped barn built on the edge of Castletown House Estate of the Conolly family, which borders Leixlip and Celbridge, Ireland. It was built in 1743 on the Leixlip side of the Castletown Estate. Flanked by two smaller dovecote towers, it was built with the stairs ascending around the exterior of the building.
Several purposes are suggested for the unique structure. One theory is based in the custom in Georgian times of using doves as a delicacy when other game or animals were not in season, and suggest its use as a dovecote. The height of the structure would also lend itself to sport shooting, supporting another theory of its use as a shooting or game keepers tower.
The tower is seen from the east windows of Castletown House, so it filled that vista, possibly as a folly.
However, a central hole through each of the floors supports the generally accepted theory of its use as a granary. The barn was built in the years immediately following the famine of 1740-41, as there was a need for new grain stores in case of another famine. The Conollys owned Kilmacredock and rented it out, so the barn was also useful for their tenants.
The construction project also likely served as a way to keep the local poor employed. In this it is not unlike the Conolly Folly (an Obelisk) which was built on the estate in 1740-41.