If you are referring to when he returns to work after staying home sick, then the irony in the first call that he answers is in the fact that the call is for his own home. This is ironic (the opposite of what is expected) because first of all, he's a fireman. So, you don't expect a fireman's house to be torched, or a fireman, who is the person doing the burning most of the time, to be the one who breaks the rules, because they know the consequences better than anyone. Also, it is his own wife that turns in the alarm; typically, you wouldn't expect such betrayal. Montag asks, "was it my wife turned in the alarm?" and Beatty nodded. It is also ironic because Montag and Faber had just gotten done planning on planting books in firemen's homes so that those firemen's houses would be burnt: "Plant the books, turnin an alarm, and see the firemen's houses burn." We don't expect Montag's home to fall to the same fate that he was plotting against other firemen's houses. The third reason it is ironic is because he actually returned to work; that is supposed to be a show of trust, a show of loyalty to Beatty and their society. As soon as he gets there, Beatty "put his hand to one side, palm up, for a gift. Montag put the book in it." So, he even returned the book. So, he's going back to work, right? He's trying to be a good citizen, right? Well, if he was, then his house wouldn't be burned.
I hope those reasons helped, and I hope that I got the right part of the book for you. Good luck!