The ironic thing about the characters is that Mr. Shiftlet is not at all interested in money or marriage until he stays with Mrs. Crawford for some time.
Irony is when something happens that is not what is expected. In this case, you would expect Mr. Shiftlet to reform Mrs. Crawford, not for Mrs. Crawford to corrupt Mr. Shiftlet.
Even though Mr. Shiftlet is a tramp with one arm, Mrs. Crawford apparently decides that he is the best her mute daughter is going to get. At first, Mr. Shiftlet seems like a great guest. He doesn’t want money to work on the farm, he fixes the car, and he teaches the daughter to talk. Yet the longer he stays, the more interested he becomes in money. Mr. Shiftlet got what he wanted—a life where he could live off the land and not worry about the material, and yet instead of it brining him closer to God he gets farther and farther away.
His interest in the car is an example of this ironic shift. At first, he just sleeps in it as a place to remain humble. Yet he soon becomes interested in fixing the car. Then he seems to become more and more materialistic.
"I can't get married right now," he said. "Everything you want to do takes money and I ain't got any."
By saying this, he convinces her to pay him to marry her daughter, and basically give him the car too. This is quite a change from the humble man he was to begin with.