What is ironic about the term "Good morning" in this book?

This is ironic because of the way that Gandalf presents it. Tongue in cheek he is playfully asking the hobbits about the phrase. For example he says: "What do you mean?" Gandalf said. "Do you wish me a good morning, or mean that it is a good morning whether I want it not; or that you feel good this morning; or that it is morning to be good on?"

Later he says: What a lot of things you do use Good morning for!" said Gandalf. "Now you mean that you want to get rid of me, and that it won't be good till I move off."