Trim Castle -  Rebuilt on the site of the earlier wooden castle.

Hugo de Lacey was pleased with his progress. He had accompanied King Henry II on his expeditions to Ireland in 1171, and for his reward was granted the kingdom of Mide (Meath), along with the city of Dublin. But his self satisfaction was to be brief. Tiernan O'Rourke, king of Breifne had been expanding his boundaries into Mide. In 1172, Tiernan met with de Lacey and was treacherously killed by de Lacey and  Donnal, the son of Annadh O'Rourke, one of his own tribe while parleying with de Lacey on the Hill of Ward in Mide. Hugo then expanded his boundaries into Tiernan's territory and set about building castles to defend his borders. He chose Trim to be his principal castle and set about planting Mide with tenants drwn from his Welsh estates. 

Roderick O'Conor observed all of this and concluded that Henry would not stop at the kingdom of Mide. It was obvious that Henry planned to continue his conquest into other Irish kingdoms. He sent messengers to other provincial kings and chieftains urging them to join him in a battle of confrontation with de Lacey in order to stop him in his tracks, and perhaps even to regain some of the land that de Lacey had taken.

The opportunity O'Conor was waiting for came when de Lacey was called back to England by King Henry. Hugh Tyrell was left in charge of the castle, which at that time was built of wood and was surrounded by a deep ditch. 

'All at once. O'Conor, The proud King of Connacht, called with him O'Flaherty, MacDermot and Mageraghty.....Towards Trim they set out marching, to demolish the castle.'

Meanwhile, Tyrell, hearing of the size of the army being massed against him, sent an urgent message for help, hoping that his courier could contact de Lacey before he embarked for England. But it was too late. de Lacey had left and it was up to Tyrell to defend the castle. He sent out scouts to estimate the strength of his adversaries, and when they reported to him that he was seriously outnumbered he decided to abandon the castle. When he saw his enemy just a few miles away, he set fire to the castle and beat a hasty retreat.  A few hours later, O'Conor arrived at the remains of the castle, which by then was just a pile of smoldering logs surrounded by a water filled ditch.

Overjoyed at having won the conflict without a battle, O'Connor and his allies returned to their own territories. 

Postscript:

In 1180, de Lacey sought to strengthen his relationship with O'Conor and married the daughter of Rory O'Conor. His alliance with O'Conor raised King Henry's suspicions that perhaps de Lacey might be aspiring to the kingship of all Ireland.  He was recalled to the Royal Court and reprimanded, but was allowed to return to Ireland accompanied by a royal clerk, Robert of Shrewsbury. Later, in 1185, King Henry sent his son John to Ireland to assert control over de Lacey, but was unable to do so. de Lacey's alliance with O'Conor made him a very powerful adversary.

In 1186, de Lacey was inspecting a castle at Darrow with three English companions. A young Irish member of his household, Gilla-gan-inathar O'Meyey approached de Lacey, and withdrawing an axe he had concealed in his cloak, beheaded de Lacey with one blow. The young man fled the scene and was never caught for his act of vengeance. When King Henry heard the news, he is reported to have been overjoyed.