Jane’s first meeting with Rochester is ironic because when he is injured she offers to help, and she does not know that he is hew new employer. He is weak and she is strong, in this case. It’s foreshadowing of their later relationship, where they defy social convention.
Irony is when something unexpected happens. In this case, it is situational irony. When Jane first meets Rochester, she has no idea who he is. At this point, he is a stranger. She causes him to fall off of his horse, and so she is at the advantage.
I was in the mood for being useful, or at least officious, I think, for I now drew near him again.
“If you are hurt, and want help, sir, I can fetch some one either from Thornfield Hall or from Hay.”
“Thank you; I shall do: I have no broken bones—only a sprain;” and again he stood up and tried his foot, but the result extorted an involuntary “Ugh!” (enotes etext p. 81).
Not that Jane is the one helping him, and offering him help. This is ironic because he has the social advantage over her, but she simply does not know it yet. She also caused the injury accidentally, and so she has him at a disadvantage.
Throughout their relationship, Jane and Rochester never behave as convention and social class suggest they should. This unusual meeting is a sign of things to come.
Rochester continues to question Jane, and he has the upper hand because he knows who he is but she does not.
“You live just below—do you mean at that house with the battlements?” pointing to Thornfield Hall, on which
the moon cast a hoary gleam, bringing it out distinct and pale from the woods, that, by contrast with the
western sky, now seemed one mass of shadow.
“Whose house is it?”
“Do you know Mr. Rochester?”
“No, I have never seen him.”
“He is not resident, then?”
“Can you tell me where he is?”
“I cannot.” (p. 82)
That he doesn’t tell her is another irony, and indication of their later relationship.
Finally, Jane’s reaction to the event is also ironic, because she dismisses it almost immediately.
I took up my muff and walked on. The incident had occurred and was gone for me: it was an incident of no moment, no romance, no interest in a sense; yet it marked with change one single hour of a monotonous life. My help had been needed and claimed; I had given it: I was pleased to have done something; trivial, transitory, though the deed was, it was yet an active thing, and I was weary of an existence all passive. (p. 83)
Her use of the word “romance” is ironic, because the event will be important. It is her first view of him, and his first view of her, entirely uninhibited by social class.