There are several ironies to this occurrence in the novel. The reason that there is so much food at the Finch household is because the black community of Maycomb are expressing their gratitude and appreciation to Atticus for doing such an amazing job at defending Tom Robinson, by bringing him food. He tried his hardest and did his best for Tom, and by extension, all of the black people in the town. He represented them and their interests well. The first way that it is ironic is that they are showing him gratitude even though he didn't win the case. Tom was found guilty; you might expect the black community to be angered at Atticus, to hold resentment and bitterness towards him because he didn't get Tom an innocent verdict. So, it is ironic that they don't; instead, they are grateful that he did such a good job of defending Tom. They were honored to be presented as they were, even if it didn't win the case. The irony exists in the fact that Atticus and Tom lost, but they still show their gratitude.
Another possible irony is that these very poor black people are paying homage to a white man. That is pretty rare for their times. And, they manage to do it during the depression, when times are very tough and food is hard to come by. You wouldn't expect them to give so much, to a white man.
The last irony that I noticed was the way that Harper Lee described the sentence. There was "enough food to bury the family." This is ironic because the real person that got buried (figuratively speaking at this point) was Tom; the guilty conviction took him away from his family and buried him in jail. You don't expect Lee to be describing the Finch family as being buried in any way, and yet she does here. As Tom is buried in jail, the Finches are buried in gratitude from his people. So, Lee phrased the description to be ironic.
I hope those explanations helped; good luck!