This is a very apt question to ask of this text. What is so ironic about this story is the way in which the author presents the madness and lack of inhibitions that comes with old age (for some) as a vehicle for Yang, the elderly and infirm tutor who the protagonist of this story is called to care for, to actually speak the truth and look at reality and see it for what it really is.
Jian Wan finds himself doubting the sanity of Yang because what he says and how he acts now stands in sharp contrast to how Yang has lived his life and his thoughts and actions for so long. For example he begins to suggest to Jian Wan that he should read The Bible and Dante's Divine Comedy, saying that these two books are able to give man comfort in his old age. In addition, he begins to make speeches against the Communist party leaders, and says to Jian Wan that he feels his life has been spent in vain because the government in China is totalitarian and only seeks to control its people rather than use study and intellectual inquiry to promote scholarship. The irony is that those around Yang dismiss his ramblings as the words of a "crazed" man, whereas the reader is able to see that his old age has brought Yang a sanity that has been absent in his working career.