It is ironic because in our way of thinking firemen put out fires rather than start them. In F451, the firemen start fires to keep people safe from the ideas presented in books. They are still protecting and serving the people, but in a much different manner.
The firemen are at times tempted to hoard or read books themselves. It is a hazard of the job. Montag steals books and attempts to read them much to the dismay of his wife. His boss, Beatty, even tells him that he may read for a day and then put the book in the incenerator after he discovers that the ideas contained in it are rubbish. Beatty is well-read and knowledgeable about literary classics, but he is the chief fireman.
The firemen have rewritten their own history to suit their own purposes. Benjamin Franklin did organize a fire brigade in the early years of the United States history, but he did not burn books, he wrote books.
The firemen use kerosene rather than water in their hoses. It is all rather backward from how we perceive the work of firemen.
We see book burning as a type of censorship and backward thinking. But in F451, book burning has become normal and a practical way to keep the population in line. The firemen have become enforcers of the social norm rather than protectors of life and property.