During the period 1170-1220 the Anglo-Normans began the Colonisation of Ireland, building Motte and Bailey Castles throughout Ireland. The wooden castle that stood on the top of the motte at Clonmacnoise was destroyed by fire and 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.
Reference: http://www.megalithicireland.com/Clonmacnoise%20Castle.html