There is some ironic humor--or, perhaps, black humor--in the fact that a good man is, indeed, hard to find; but, a bad man is very easily encountered. With her purse-full of platitudes the grandmother has a rather patent faith that does not clarify itself spiritually until she encounters the bad man who acts as an agent of grace for her.
The fact is, then, that the grandmother has an epiphany when she meets the Misfit and recognizes her kinship to him as a sinner and is, thus, saved. So, until she encounters the Misfit, all the good men who are difficult to find serve her not for her salvation. Indeed, after she dies, the Misfit notes,
"She would of been a good woman if it had been somebody to shoot her every minute."
He, then, is the agent of grace for the grandmother, not any "good man."
The irony of the title is shown in the way that the grandmother uses the phrase "a good man" in the story. She uses it twice: firstly in her description of Red Sam and secondly when she meets the Misfit. However, upon closer examination, it becomes clear that the grandmother defines a "good man" rather curiously. For example, with Red Sam, she calls him a "good man" after he explains how he let himself be cheated out of gasoline by two men. Being "good" in this context seems to indicate being gullible and also nostalgic for the past, both of which are two aspects that the grandmother herself can strongly relate to.
Secondly, she calls the Misfit a "good man" because she desperately tries to appeal to the fact that because he does not have bad blood he would not shoot a lady like herself, no matter what else is happening that might suggest otherwise:
"I just know you're a good man," she said desperately. "You're not a bit common!"
This is erroneous for at least two reasons: firstly, she bases this claim on her mistaken belief that the Misfit comes from a good background, and secondly, she is judging the Misfit by her own moral code. Both of these assumptions prove to be mistaken. What is ironic therefore about the title is that "good" is shown to not relate to "moral" or "kind," as the reader would normally define the word. Instead, "good," as the grandmother uses it, is a word that relates to her own moral code, which is of course very different from the moral code of those around her. It is the way that she judges others by her moral code that results in the tragic denouement of this short story.