At the time of The Battle of Clontarf, the Clontarf Castle area was wooded with a river flowing by. Many of the rivers in the history of Dublin no longer flow through the same area, as they were re-routed when the city was walled.
It all began even before Clontarf Castle was built, when Mael Morda, King of Leinster, began to plot against Brian Boru. Mael Morda made an alliance with Sitric, the Viking King of Dublin, who was assisted by the Vikings of the Orkney Islands and the Isle of Man. Brian Boru marched against them and the great battle was fought at Clontarf. It ended in victory for Boru’s army. However, on the night of the victory, Boru was praying in his tent, surrounded by five men, who were guarding him. A small group of Vikings, who were retreating from the battle through a wooded area, close to the site of what is now Clontarf Castle, came across the guarded tent. Realizing who was being protected; they killed all five guards and went on to kill Brian Boru, who by now was 72 years of age.