Built in the late 15th or early 16th century from limestone rubble with carefully cut corner stones, this five-storey structure measures approximately 10 metres east to west and 12.7 metres north to south. The Civil Survey of 1640 records that Nicholas Everard of Fethard owned the property, which then comprised ‘a Castle & a slate house within a bawne’. The tower’s original design featured flat, ogee and round-headed windows with chamfered edges, a vault over the main third-floor chamber, and defensive features including a machicolation above the entrance and a murder-hole in the chamber above the lobby.

Around 1600, the castle underwent significant remodelling that transformed its character whilst preserving its defensive capabilities. The original windows were replaced with fashionable two-light mullioned and transomed designs, and multiple fireplaces were inserted throughout the structure, including a particularly fine example in the fifth-floor mural chamber with carved inverted pyramidal jambs. The renovation also added distinctive five-sided chimney stacks that appear rectangular from inside but diagonal externally; an unusual architectural feature that demonstrates the creativity of early modern Irish builders. The tower’s complex internal layout includes numerous mural chambers accessed via a spiral staircase in the southwest corner, with garderobes, slop-stones, and wall cupboards indicating the domestic comforts expected by the gentry of the period.

Later alterations, possibly in the 18th century, saw a house built against the south wall of the tower, the roof scar of which remains visible today. This addition caused considerable damage to the original entrance arrangement, with new fireplaces inserted where the doorway once stood. Despite these changes and the current state of partial ruin, with some upper floors now inaccessible, Knockkelly Castle remains a remarkable example of how Irish tower houses evolved from purely defensive structures into more comfortable residences whilst retaining their fortified character. The building showcases the layers of Irish history, from its medieval origins through Renaissance improvements to Georgian additions, all visible in the fabric of this impressive structure.