There are two major ironies in "How Much Land Does a Man Need?" Irony is a contrast between two things. In this case there is dramatic irony--a contrast between what the character knows and what we, the readers, know--and situational irony--a contrast between what we expect to happen and what actually happens. The dramatic irony is that Pahom is so confident of his position that he taunted the Devil, and it's the Devil who tricked him and causedhis death. Pahom claimed that if he just had enough land he wouldn't be afraid of the devil himself. When he is offered the opportunity to finally get "enough" land, the Devil (in disguise, of course) comes to him in a dream, but Pahom dismisses it. The next day his greed causes his death. He taunted the Devil, and the Devil won. The situational irony is the central theme of the story and is best represented in the title question: how much land does a man need? Pahom thought he would be happy if he just had enough land, and he gathers more and more as he grows older. But it's never enough; he seems to have an insatiable hunger for more land. In the end, the only land he truly owns is the only land he actually needs--six feet of land on which to be be buried.
I believe you're referring to the story "How Much Land Does a Man Need?" by Leo Tolstoy. In this story, Pahom is the protagonist, or main character.
In the story "How Much Land Does a Man Need?," Pahom desires nothing more than land. After purchasing forty acres of land, he and his wife are content until he can no longer tolerate the trespassing of his neighbors on his wheatfields and their allowing their cattle and horses to eat his crops. Pahom endures this treatment for some time before leaving for land "beyond the Volga," where he is granted 125 of communal land.
When Pahom runs into difficulties with a farming partner, he begins to look for land that he could buy. A dealer who is passing through tells Pahom of the "land of the Bashkirs, far away, where he had bought thirteen thousand acres of land, all for a thousand rubles." Pahom, of course, immediately sets out for the place.
After Pahom has eaten with the Bashkir and given them gifts, the interpreter informs him that the people will return his kindness by presenting him with whatever they possess that pleases him most. Pahom chooses their land as his gift. The chief tells Pahom that
Our price is always the same: one thousand rubles a day...We sell it by the day. As much as you can go around on your feet in a day is yours, and the price is one thousand rubles a day...But there is one condition: If you don't return on the sae day to the spot whence you started, your money is lost.
In his greed an worry over issues that do not matter (the shape of his piece of land, etc.), Pahom pushes himself too hard.
He was exhausted from the heat, his bare feet were cut and bruised, and his legs began to fail. He longed to rest, but it was impossible if he meant to get back before sunset. Pahom runs on and on, far exceeding his physical limitations. He reaches the chief just as the sun sets. However, he falls dead at the chief's feet.
The final sentence of the story sums up its irony: "Six feet from his head to his toes was all he needed." In other words, Pahom's greed and desire for land drove him to his death. He got exactly what he wanted, but he never got to enjoy it, because he died in the process.