The 19th Century OS Map shows clearly a Norman tower house (castle) and a later structure stand close together on the site. From a look at the Aerial OS Map it seems that the tower house is still sanding and visible on the landscape though the later house isn't.
Some local folklore about the Castle from the Schools' Folklore Project c.1937 states that:
There is a supposed Sacred Room in Knocka Castle, one of the most ancient buildings in the parish of Templemore. There was a large quantity of Irish treasures and ancient Irish books which were stolen by Cromwell, a suit of arms was also found and a picture of Cromwell and a frame of gold were found and sent to the British Museum. There is a secret passage from Knocka Castle to Barnane Castle where our priest often hid when they were persecuted for their religion.
References
Duchas Schools' Collection | Ireland | VIEW SOURCE |