I had this same question as a prompt for an excessively long essay that I had to write a while back. It was difficult because when we think of irony, we might think of socratic irony, when ignorance is intended in a discussion, dramatic irony, when we as readers know more than the characters, or perhaps verbal irony, usually referred to as sarcasm. There is another kind of irony that is present in the short story Marigolds. This kind of irony could be as simple as a surprise ending or twist in the plot.
When Elizabeth trashes the prized marigolds in Miss Lottie's garden, she is not angry, when readers asume and definitely expect her to be. This is called situational irony.
Another place that is a bit ironic is at the end when the narrator says "And I too have planted marigolds" (Collier 84). This is ironic because Elizabeth has grown out of her innocence and developed compassion for Miss Lottie and for the reason that she planted them in the first place. We as readers never would have expected her to want to plant marigolds of her own one day.
I would suggest reading the story over again and looking for things like exaggeration, sarcasm, and understatements, which are all types of verbal irony, too.
Hope this helped :)