Built from randomly coursed granite with striking white dressed granite quoins marking its corners, the structure exhibits the defensive architecture typical of medieval Ireland. The tower features a slight base batter for additional stability and a buttress supporting its eastern wall, whilst its thick walls, measuring 1.2 metres, speak to its original defensive purpose.

The interior reveals both the tower's original medieval character and later modifications. Entry through the northern side of the western wall leads to a rectangular chamber measuring 10.1 metres long by 5.5 metres wide. A stair turret occupies the northeastern angle, its ascent illuminated by narrow slit loops. The ground floor's barrel vault shows damage from later insertions of fireplaces and ovens into the northern wall, whilst deep window embrasures pierce both the eastern and western walls. The first floor retains more of its residential character, with plastered walls, a fireplace in the western wall, and notably different window styles; a double-light granite window with chamfered lintel and a triple-light limestone window, suggesting renovations across different periods. A small garderobe tucked into the southeastern corner provided medieval sanitation.

The castle's turbulent history came to light through various discoveries around the site. During Cromwell's campaign through Ireland, Shankill Castle found itself under siege, leaving its mark on both the structure and the surrounding landscape. Excavations have yielded coins from the reigns of James I and Charles I, placing the castle firmly within the early 17th century conflicts. More grimly, skeletal remains discovered in the castle's environs serve as stark reminders of the violence that once visited this now peaceful corner of Dublin.