John Steinbeck is a very clever writer. Hence, he uses irony in his book in various ways. Some of them are clear and in your face; others are subtle.
One of the clear uses of irony emerges right way. Steinbeck gives the biggest man in the book the name of Lennie Small. There is nothing small about Lennie. He is a hulk of man, and he shows it in the fields as he works and when he crushes Curley's hand in a fight.
Another clear use of irony can be seen in Curley's wife. She comes onto all the men and she is attractive, but all the men turn her away and shut her down.
A more serious use of irony is the death of Lennie. When Lennie accidentally kills Curley's wife, George knows that the only fate left for Lennie is a slow death by an angry mob of men. Therefore, as his best friend, he has to find Lennie before the others do. When he finds him, he tells him the story of how they will live off the fat of the land and then he shoots him in the head. George's love and loyalty to Lennie made him take his life. This type of irony can only make the reader shed a tear at the hardships of migrant workers.
Also I should say that not all ironies are created equally. Some are funny, others are clever, and still others are tragic.