What is the sad irony in this sentence from Nectar in a Sieve? "When Kuti was gone - with bland indifference that mocked our loss - the abundant grain grew ripe".

Only a few weeks after Kuti dies, a victim of starvation, the grain finally grew ripe "with a bland indifference that mocked (Nathan's and Rukmani's) loss".  It was an exceptionally good harvest, completely unexpected;

"Every husk was filled, the paddy stood firm and healthy, showing no breaks in their ranks".

The harvest yielded

"neat white hills of rice and the husks in a rustling brown heap...enough to pay what (they) owe...and to keep what (they) want".

Nathan and Rukmani had not experienced such wealth and plenty in a long time, if ever.  It is sadly ironic that they are blessed with such richness just a little too late - after their young son has died because, only two weeks earlier, they had nothing (Chapter 17-18).

Irony, and specifically situational irony, is when things turn out the opposite of what is expected.  In this case, the characters never expected their fortune to change so completely, and it is especially tragic, that when it does it is just a little too late to save their youngest child.