Holy cow: 3 sentences! This story is worth much more because of its intensity of emotion and its final disappointment.
I think the flashback occurs when the narrator returns to the room where the priest had died, and the narrator retreats into himself.
There are several examples of foreshadowing; you'll have to take your pick.
Early in the story the narrator says
[Mangand's sister's] dress swung as she moved her body and the soft rope of her hair tossed from side to side.
This is such a strong hint of the narrator's infatuation that it made me sit up and take notice. I could imagine how the story was going to proceed.
Earlier in the same paragraph the narrator says
If my uncle was seen turning the corner we hid in the shadow . . .
Already we suspect the uncle is going to be the narrator's obstacle.
The narrator's infatuation grows rapidly, and Joyce paints it vividly. I think a better topic for studying this story would be to examine Joyce's imagery.
Foreshadowing again: on Saturday morning the narrator reminds his uncle of the fair, the the uncle replies
"Yes, boy, I know."
So we are not surprised when the uncle returns almost too late in the evening for the fair to still be open.
I suppose that's plenty for three sentences. I'm afraid you'll have to trim it a bit. This story deserves much more attention.
The reference gives the text of the story.