This tall rectangular fortress, measuring approximately 11 metres east to west and 7 metres north to south, first appears on historical records in the 1654-7 Down Survey map of Killare parish, where it's marked as 'Killinbracke' on the property of Arthur Nangle, noted as an 'Irish papist'. The survey's terrier confirms that several castles stood in the area at the time, including those at Clare, Dooneild, Rathskeagh, Bishopsland and Ballikenny, suggesting this region was once densely fortified.

The castle's architecture tells a story of both medieval defence and later domestic comfort. Built from limestone rubble with a slight base batter, the structure rises four storeys high with its entrance positioned at the northern end of the eastern wall. Just inside, visitors would have encountered a small stone-roofed guardroom, whilst a circular turret in the southeast corner houses the remains of a spiral staircase that once provided access to the upper floors. The defensive nature of the tower is evident in the gun loops flanking the windows on the north and west walls, whilst the upper levels featured mullioned windows and fireplaces served by a distinctive diamond-shaped double chimney stack that still rises above the roofline.

During the 17th century, the castle underwent significant expansion with the addition of a substantial southern wing measuring 12 metres north to south and 11 metres east to west, though only fragments of its corners and western wall survive today. This extension formed part of a larger defensive complex that included a rectangular bawn measuring 52 by 42 metres, of which the southwest corner remains intact, complete with a circular flanker tower and adjoining walls that still stand three metres high. Single-storey buildings also extend eastward from the main tower, though their exact date remains uncertain. Despite the robbing of many lower quoin stones and damage to most windows, Killeenbrack Castle remains an impressive testament to the tower house tradition that once dominated the Irish countryside.