Definitions of irony on the Web--which indicate to me that the purity of sense to which McGrath appeals is long since lost:

witty language used to convey insults or scorn; "he used sarcasm to upset his opponent"; "irony is wasted on the stupid"; "Satire is a sort of glass, wherein beholders do generally discover everybody's face but their own"--Johathan Swift
www.cogsci.princeton.edu/cgi-bin/webwn


incongruity between what might be expected and what actually occurs; "the irony of Ireland's copying the nation she most hated"
www.cogsci.princeton.edu/cgi-bin/webwn


a trope that involves incongruity between what is expected and what occurs
www.cogsci.princeton.edu/cgi-bin/webwn


saying [or writing] one thing, whilst meaning the opposite
www.mantex.co.uk/samples/eng.htm


expression that comes across contrary to the intended meaning, often because the audience knows what the speaker does not.
rinkworks.com/words/linguistics.shtml


In literary criticism, the effect of language in which the intended meaning is the opposite of what is stated. The title of Jonathan Swift's "A Modest Proposal" is ironic because what Swift proposes in this essay is cannibalism — hardly "modest."
www.galegroup.com/free_resources/glossary/glossary_im.htm


stating something by saying another quite different thing, sometimes its opposite. An example is Sir Thomas Wyatt's "And I have leave to go, of her goodness" from his "They flee from me."
www.creativestudios.com/lit/glossary2.html


a method of expression in which the intended meaning of the words is the direct opposite of their usual sense. Example: "The speaker was using irony when he said that the stupid plan was 'very clever.'" Irony can also mean a combination of circumstances or a result that is the opposite of what might be expected or considered appropriate.
www.humanities.eku.edu/Glossary.htm


the difference between how you might expect something to be and how it actually is, for example when the slaves in The Two Generals like the brother who believes in slavery more than the one who would set them free
www.longman.co.uk/tt_seceng/resources/glosauth.htm


The mythos (sense 2) of the literature concerned primarily with a "realistic" level of experience, usually taking the form of a parody or contrasting analogue to romance. Such irony may be tragic or comic in its main emphasis; when comic it is normally identical with the usual meaning of satire.
www.sil.org/~radneyr/humanities/litcrit/gloss.htm


At its most basic, a difference or gap between the presentation/representation of something and its reality. In other words, when what something appears to be and what it is are not the same. Irony can be engaged or detached: Engaged irony uses the gaps between reality and representation to make a point or expose something; detached irony exploits gaps for immediate effect, like humor, satire or surface criticism. Irony can also occur at different levels of a text; for instance, verbal irony would occur at the level of the word or sentence, where double meanings come into play; dramatic irony would occur at the level of the plot, where events and action are constructed in a way to take the reader in one direction while the reality is something else (a technique often found with 1st person unreliable narrators and 3rd person privileged narrators).
www.viterbo.edu/personalpages/faculty/jwood/vocabulary%20page.htm


the use of words to convey the opposite of their literal meeting
ww2.aps.edu/~apsedumain/CurriculumInstruction/glossary.htm


The use of language to express something quite different from or opposite to its literal meaning.
fajardo-acosta.com/worldlit/glossary.htm


The use of words to convey the opposite of their literal meaning; an expression marked by such a deliberate contrast between apparent and intended meaning; incongruity between what might be expected and what actually occurs.
www-relg-studies.scu.edu/facstaff/murphy/courses/sctr032/glossary.htm


techniques that involve surprising, interesting, or amusing contradictions
www.geocities.com/educationplace/poe/gl.htm


this means that something is the opposite of the meaning of the words, for example 'a fine mess'. 'Fine' usually means something good, but in this case it means a bad mess.
elc.polyu.edu.hk/advdicts/glossary.htm


a difference between the actual result of a sequence of events and the expected results
www.brawleyhigh.org/writingstyleguide/glossary.htm


a situation or statement characterized by a significant difference between what is expected or understood and what actually happens or is meant. See cosmic irony and dramatic irony. Close Window
www.wwnorton.com/introlit/glossary.htm


The suggestion of the opposite, or nearly the opposite, as in saying that being caught in a freezing downpour is "delightful."
www.womens-studies.ohio-state.edu/pedagogy/Writing/glossary.htm


is a result different from the expected.
www.homepages.dsu.edu/jankej/Writing/glossary.htm


A literary device that uses contradictory statements to reveal a reality different from what appears to be true.
ctl.clayton.edu/English/glossary.htm


, saying something, but meaning the opposite. E.g., "that Mustang Cobra is really bad." Usually irony is not deceptive; the audience is supposed to realize what the speaker really means. But irony can also be used to keep some of the audience, those who don't "get it," in the dark. Because Socrates used irony this way, he is often called "the ironic man," famous for "Socratic irony." Some have argued that Paul also deserves the title.
courses.smsu.edu/mdg421f/reli321/glossary.htm


A device by which a writer expresses a meaning contradictory to the stated or ostensible one. (used to achieve special rhetorical or artistic effects)
www.english.uga.edu/cdesmet/class/engl4830/work/projects/brent/chardef.htm


[noun]: incongruity between what might be expected and what actually occurs [Characterization]
www.southlakems.org/AnimalFarm/DanielleAmanda/glossary.htm


two separate and contrasting levels of meaning embedded in one message
www.migrant.org/assets/literature/literary_glossary.cfm


a general definition: suggesting more than is actually said saying one thing and meaning another (verbal irony) seeing contradictions between the way things appear and what they really represent (an ironic world view), and exposing those contradictions (satirical irony) recognizing that human beings are nothing more than the playthings of fate or God - revealing the Catch-22 nature of human existence, such as the habit of striving for an ideal which cannot be met (cosmic irony)
gs.fanshawec.on.ca/online/engl286/glossary.htm


A striking contrast between the apparent and the real situation or between what a character says and what the reader knows.
www.crown.edu/humanities/Wheatonj/eng132/Terms%20-%20Short%20Stories.htm


the use of words to express something different from and often quite opposite to their literal meaning
www.oswego.edu/~thoffman/semiotics/assignments/assignment04/activity_1.html