One example of irony at the end is the method Montag uses to escape. For his whole adult life, he has been a fireman, using kerosene to burn books and houses, using fire to destroy. Earlier, he wonders if firemen used to put out fires instead of setting them, and he discovers that the revised history of the world has erased this truth. Now, on the run from the Mechanical Hound, Montag can't use fire to save himself. Instead, he uses water:
He floated on his back... The river was very real; it held him comfortably and gave him the time at last, the leisure, to consider this month, this year, and a lifetime of years.
(Bradbury, Fahrenheit 451, Google Books)
As a fireman, he should have been using water to put out fires. Now, water saves him from the Mechanical Hound and all the technology of the New Fire Department. His thoughts on the river are the final nails in his understanding of fire as a destructive force, and he understands -- because the water gives him time to think -- that since time will not stop its slow destruction, humanity needs to stop destroying and start to educate and build.