Please explain the use of irony.

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.