In the first paragraph of his story "Killings," Dubus introduces readers to the Fowler family. They have not gathered for a happy occasion, but for the funeral of the youngest son, Frank. So the symbolism we see in the opening paragraph is that of a funeral: the limousine, the casket, the grave, the minister, the pallbearers, the grieving family. As for irony, I really don't see anything ironic in this first paragraph, unless you consider that the apple trees are lined up symmetrically, just as grave markers are lined up in a cemetery. But that would be more imagery than irony. Perhaps it is ironic that Steve, the oldest Fowler son, says to his father, "I should kill him." At this point in the story, the reader doesn't know who the "him" he's referring to is. So it could be ironic that he is talking about killing someone when he's standing near a fresh grave.
I hope this helps you.