The witches' words to Macbeth suggest that he shall have a long reign as king, especially considering the word "hereafter," which means "from now on." However, their prophecy to Banquo is that he will be the father of kings. We quickly learn that Banquo already has a son. It would be against tradition for Macbeth's throne to go to Banquo's son years down the road - one would assume that Macbeth would have his own children by the that time to pass his crown to. Therefore, Macbeth's prophecy is ironic because it seems to be contradictory in light of Banquo's. It is also ironic because the prophecies give Macbeth (the murderer) one reign while giving Banquo (the murdered) many.