In what way is Duncan's treatment of Macbeth in Act 1, scene 4 and Act 1 scene 6 an example of irony?
The irony in this scene is that Duncan tells Macbeth that he owes Macbeth more gratitude for his valiant efforts in the recent battle than he can possibly pay him. Macbeth says then that just being able to serve Duncan is payment enough. Next Duncan tells Macbeth that while he has given him one reward (the title of Thane of Cawdor), that's just the start of his rewards. Then Duncan names his son, Malcolm, as his successor, the Prince of Cumberland. Macbeth was just told in the previous scene by the witches, that he would become king when they greeted him with the title "King". He'd also been greeted as the Thane of Cawdor which title he received almost immediately after the witches greeted him as such. It makes sense then that after having done so well in battle and being a male relative of Duncan's that he might get named as successor. Macbeth, disappointed, immediately realizes that he may have to make it happen if he is to become king. Duncan continues to praise Macbeth, calling him a "peerless kinsman" which is ironic because it is indeed rare for one relative to kill another and Macbeth is certainly rare in his extreme ambition and lack of conscience we will come to see.