The irony of savage associations
To take a charming, folkloric community like the Southern New Jersey peoples and to compare them to savages is ironic. It's mostly ironic because the heinous crimes that occur in normal cities are way more extreme and violent than what happens in the woods. The poor association has its origins in unfair biases.
The irony of fire
Fire is necessary for growth and balance, but without management, too much damage can be done. There are biological and sociological components to fire management, but most sad of all is when human negligence leads to the loss of life. Ironically, though, fire can be a natural and normal part of the life cycle of the forest.
The irony of progress
Progress is one of the central issues of the book, but instead of showing an immature or non-developed community, McPhee shows that the lack of development has led to the development of a sweet culture and an earthy, meaningful way of life.
The irony of crime
Crime is a problem in the woodlands occasionally, but even though people associate crime with the Pines, it's almost never a Pines native who commits serious crimes. In fact, the police ignore Pines citizens almost completely, because they are often isolated and harmless. The real crime is done by people who drive into the woods for the purpose of cover.
The irony of nature
It's so easy to read about communities with different priorities than their urban counterparts and to assume that everyone must hate life there. But actually, life in the Pines is rich and rewarding. They're connected to the earth, which gives them daily adventures in the form of hunts and gardening. One could make the argument that perhaps their lives are more suited for humans than the modernistic urban mode.