The castle (which, while immense, is not visible from the road) has an unusual round Keep with four square flankers (towers). Although the castle is in ruins, the entrance portcullis is perfectly preserved. Early references to the castle appeared in the mid-15th century, but it is believed that the castle itself predates even that. The remaining walls, at least 12 feet thick and constructed in a perfectly circular manner, suggest the sheer size of the original structure; doors, passage-ways, archways and underground chambers are visible. The castle was aligned with the Fitzgerald family and is situated on the border between their land and that of the Duke of Desmond. It was attacked by Cromwell in the 1650s, but survived the assault. The demesne sports a number of trees that are not common in the area, such as linden, red chestnut, copper oak and derivatives of the maple genus. The castle farm was owned by a family from Devon, the Drews, until 1925/26. Inscriptions on the gate lodge and the former National School (OMB 1882 and OMH 1884, respectively) are believed to refer to Olivia Marie Drew – one of the daughters of the family – who married into the Barry family and then, later, the Hilliards. Mocollop Castle is on private property and is not accessible by the public.