For example, your friend asks whether you've repaired the lawn mower. You respond as though your friend were referring to a hired hand who mows your lawn, saying something silly like, "He seems perfectly healthy to me."
Is that irony? I've always had trouble grasping the concept.
The American Heritage® Dictionary of the English Language, Fifth Edition, in a usage note, describes the meaning of 'irony':
In its nonliterary uses, irony often refers to a perceived incongruity between what is expected and what actually occurs, especially if what actually occurs thwarts human wishes or designs.
(ironic. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved September 30 2015 from http://www.thefreedictionary.com/ironic)
That sense as given could be applied literally, trivially, and misleadingly, to arrive at the conclusion that your response to your friend is an example of irony (an ironic response). Your response will almost certainly be perceived as incongruous: it is not the expected event, but occurs anyhow. Your response also, presumably, thwarts your friend's human wish for a sensible answer. So, your example meets, on the face of it, the requirements for being ironic, as stipulated by the usage note from the American Heritage® Dictionary.
However, the usage note continues:
People sometimes misuse the words ironic, irony, and ironically, applying them to events and circumstances that might better be described as simply coincidental or improbable, in that the events suggest no particular lessons about human vanity or folly.
(op. cit.)
So, is your response to the question incongruous, or merely improbable? As it stands, without additional context such that might connect the apparent incongruity of your response with the question that elicited the response, your response seems to me to be merely an improbable event: it is improbable that you would offer such a response as a response to the question. Beyond that no incongruity is evident.
Your response is, however, typical of a literary and conversational device called a non sequitur: your response does not follow from the question. This device, the non sequitur, frequently has a humorous dimension, whether the paucity of logical connection between the non sequitur and what preceded it is intentional or unintentional.
The humor of a non sequitur, as is often the case with humor, defies analysis, but seems to arise from absurdity, in the sense that a non sequitur is
b. Impossible to take seriously; silly.
(absurd. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved September 30 2015 from http://www.thefreedictionary.com/absurd)
Irony requires an incongruous contrast. Often the contrast is between a situation and the language used to describe it. For example, suppose an politician is given a rousing introduction to an audience that greets him with silence. When the politician says
Thanks for your enthusiastic support
he's being ironic because his words clash with the situation in which he gets no support at all. The effect need not be deliberate. As an example of so-called dramatic irony, in Sophocles' play Oedipus, the title character promise to track down his father's killer. The irony arises form the incongruity that Oedipus is unknowingly talking about himself. The audience is aware of this, but Oedipus isn't.
The word extends to situations in which the outcomes don't meet expectations. In December 2014, the lame duck US Senate, controlled by the Democratic Party for just a few more days, got the extra time to confirm an extra dozen federal judges nominated by the Democratic President because Republican Senator Ted Cruz kept the Senate in session over a weekend. Cruz intended to use the extra time to force a vote on an issue that he thought would embarrass the Democrats, but all he did was allow his opponents to confirm nominations that he himself opposed. Everyone noted the irony.
There doesn't seem to be enough incongruity between a lawnmower and a lawn mower to qualify for irony.
After I posted this question, I stumbled across an entry in wiktionary. I know you're not supposed to trust it, but this at least sounds plausible: Ignorance feigned for the purpose of confounding or provoking an antagonist; Socratic irony.
You have to get the opposite of what you think or expect if something is ironic. I don't think replying with "he" instead of "it" is something opposite of your expectation. It is rather surprising as if it were spoken by a mentally insane or retarded person. It is not a humor, either.
Here are some examples of irony in Merriam-Webster.
“What a beautiful view,” he said, his voice dripping with irony (atually, he is not looking at a beautiful scene), as he looked out the window at the alley.
She described her vacation with heavy irony (she is resting, not STUDYING) as “an educational experience.”
It was a tragic irony (he got sick by his own worries) that he made himself sick by worrying so much about his health.
Your case is just one of many misunderstandings that can happen in your life.
