Does the last line of the story seem ironic?

Yes, I think the line in question is very ironic, especially when you keep in mind that it answers the question that is asked in the story's title. From that perspective, this closing line tends to hover across the entire length of the story, informing everything that came before it.
Ultimately, at its heart, "How Much Land Does a Man Need" is a story about greed, with its main character, Pahom, driven by a constant need to increase his own land holdings. Of course, it is this same greed that ultimately kills him. Tolstoy thus provides an answer to the question which his story's title poses: a man only needs land enough to serve as a grave.
This is a cutting reversal to the arc of the story thus far, where Pahom has been driven by land greed, continually looking to seize any and all opportunities. From Pahom's perspective, there is no real answer to this question, because there is no minimum amount of land that would suffice for him—he's always grasping for more than what he currently has. Tolstoy, on the other hand, has a very different answer to this question, one which would invalidate everything Pahom has spent his life trying to achieve.