Answer:
By that statement, Macbeth makes an ironic comment by cursing those who trust the witches which he had been doing since the prophecy was told to him.
Explanation:
Irony is when an expected event or result did not happen but rather the opposite happens. In simple words, the irony is when one thing is expected but the complete opposite happens. So, the use of such language that is supposed to mean one thing but the opposite happens is known as irony.
The irony in Macbeth's statement "dam n ed all those that trust them" in Act IV scene i is that he is criticizing those who trust the three witches and their words. But we know his own actions were all based on what the witches had prophesied. He never had any intention of becoming the king of Scotland until the three witches told him about his future in Act I. And since then, he had made sure to try to make or see the prophecy about his accession come true, even to the point of murdering those who pose an obstacle in his way.
And just before he made that ironical statement, he was seen conferring with the witches about the fulfillment of the prophecies that they'd made. And by cursing those who trust the witches and their words, he is also unknowingly criticizing his own self.
