"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." what is ironic about the dog's response to the woman in "ah, are you digging on my grave? " by thomas hardy? he passes by her grave everyday. he misses her deeply and has come to see her. he has actually forgotten where she is buried. he is the only one who remembers her.

The right answer is "he passes by her grave everyday".

The irony in this case is that the dog speaks in the text that it passes by that place every day, soon after it says that it forgot that that is the lady's tomb, but if the dog goes there every day it knows the place very well, so it knows where the graves are and who owns them.

It is important to point out that irony is a figure through which the opposite is said of what one wants to imply.

the audience knows that nora is intentionally failing to dance correctly, but helmer doesnt