According to the dictionary, "irony is defined as incongruity between a situation developed in a drama and the accompanying words or actions that is understood by the audience but not by the characters in the story".
The character of Mr. Shiftlet, the drifter who comes upon Mrs. Crater and Lucynell, is wandering around looking for a place to find meaning in his life. He is alone, empty, devoid of any relationships. When he meets the old woman and her daughter, he gets very close to being a part of something, a family.
Mr. Shiftlet seems to be finding a place to fit in, with Mrs. Crater and her daughter, he also gives the old woman and her daughter the opportunity to have more meaning in their lives, all three are made better because of their relationship. They are "saved" from a life of loneliness and emptiness. However, ironically, Mr. Shiftlet abandons Lucynell when she passes out at the diner. The other people in the diner, and the reader, recognize Lucynell as Mr. Shiftlet's salvation, identifying her as an angel. However, he does not see it. He runs away from her and leaves his saved life behind.
Mr. Shiftlet starts at the beginning again when he gets into the car and picks up the hitchhiker, he is once again trying to find meaning in life. Ironically, he abandoned just that when he walked away from his wife and his life with Mrs. Crater.