irony in the words "look on my work mighty and despair ".

Julius Caesar was a powerful Roman emperor. He bought glories any victories to the nation. Due to his powers and work, he was very famous among the people.
Till last breath he was being boastful about his powers and his contributions towards the nation. He told the assassinators that the future rulers would never be able surpass his glory. But his boast was ironic as his over-confidence led to his death. He was highly credulous which resulted in his assassination.