A main point of irony is that King Duncan rewards Macbeth for his loyalty by granting him the title of Thane of Cawdor and this contributes to sealing his doom. Macbeth has already heard the prophecy from the witches that he will become Thane of Cawdor when he is granted the title and this gets him thinking that he can fulfil the other parts of the prophecy and become king. Duncan is standing in the way of Macbeth's ambitions.
When Duncan arrives at Macbeth's castle in Inverness, he says 'this castle hath a pleasant seat' which is an example of dramatic irony as the audience knows Macbeth's intentions to kill the king, but Duncan is unaware that this pleasant place will actually be where he dies.
In some ways, Duncan contributes to his own death by going to the castle to celebrate victory with a loyal soldier who he has inadvertently rewarded with one of the titles Macbeth has been prophesied to receive and which helps to bolster his thoughts about taking the throne.