How does this final stanza of “ah, are you digging on my grave? ” use irony to disappoint the speaker’s expectation?

In this poem, we encounter a conversation between a dog and a corpse buried underground. The final stanza reads:

"Mistress, I dug upon your grave

To bury a bone, in case

I should be hungry near this spot

When passing on my daily trot.

I am sorry, but I quite forgot

It was your resting-place."

The irony in this poem is that the dead woman was hoping someone was digging on her grave because they were thinking of her. However, not only was it a dog and not a person who did it, but the dog did not even remember she was buried there.