During the period 1170-1220 the Anglo-Normans began the Colonisation of Ireland, building Motte and Bailey Castles throughout Ireland.
In 1214 the wooden castle that stood on the top of the motte at Clonmacnoise was destroyed by fire.
Later in 1214 the Justiciar of Ireland, Henry of London, built a stone castle on the motte.
This was to gain control over the midlands and guard the bridge across the River Shannon.
This was a very significant period in history as it marks the time when Clonmacnoise went into decline as a monastic city.
In 1216 Henry of London was ordered to compensate the Abbot for his lands and any others items like animals or fruit trees the Abbot may have lost.
There is a large bank and a very deep fosse surrounding the D-shaped Motte.
The Castle was destroyed during the Gaelic Resurgence in the late 13th to early 14th century.
Originally the castle had three stories but very little of the castle remains today.
The ruins are very dangerous and delicately balanced on the edge of the mound.
Reference: http://www.megalithicireland.com/Clonmacnoise%20Castle.html