During Easter Week he was in charge of the garrison in the South Dublin Union. His second in command was Cathal Brugha. On Thursday of Easter Week, there was some confusion and after many hours of heavy bombardment a mistaken order to retreat was circulated among the troops. Brugha was badly wounded and lay unable to leave. Ceannt was mistakenly told that Brugha was dead. Brugha weak from loss of blood continue to fire upon the enemy and then suddenly the Volunteers heard the voice of Brugha singing "God Save Ireland".
In one of the most dramatic scenes of Easter Week, Eamonn Ceannt"crept on bended knees to the side of this comrade. He found him lying in a pool of his own blood. The two men embraced and Cathal said "Let us sing 'God Save Ireland', Eamonn. Then he collapsed. But he had held up the enemy's advance for 4 hours."