I thought I’d share my response:
To answer your question, I went to two sources:
2. Paul Hemetsberger, a guy at or very near the top of my list of favorite geeks of all time.
“Me” says – “There’s no difference. Use them interchangeably.”
PH (or, more precisely, PH’s online dictionary) suggests there are nuances.
PH is the moderator for a perfectly marvelous online dictionary, aka “dict.cc.” It’s an open source dictionary, free to the public, and dependent on their contributions, which he organized and now moderates, apparently from his home in Vienna.
According to dict.cc, irrsinnig covers the semantic range of English “insane, crazy, lunatic” or “stupid,” and, if used together with komisch or witzig (funny), covers the English “zany,” and “hilarious.”
Wahnsinnig covers the semantic range of English “insane, demented, delirious, mad, crazy, incredible, manic, paranoid, bedlam, psychotic, raving, mind-boggling” and “frantic.”
and, when used as an adverb, together with beliebt, komisch, langsam, schlecht, or teuer
(also komisch, witzig) translate to wildly (popular), hysterically, uproariously (funny), excruciatingly, extremely (slow), and ruinously (expensive), respectively.
Now what to make of this “difference.” That is, is it a difference that makes a difference?
dict.cc lists the combinations irrsinnig + komisch/witzig and wahnsinnig + beliebt, komisch, langsam and schlecht, suggesting a different semantic range, BUT, a quick google check will reveal that both irrsinnig and wahnsinnig work with all of the words on the list, i.e., that they are completely interchangeable. The word combinations on dict.cc seem to be random, in other words, a product of multiple sources of entry, perhaps.
Next question in a search for difference would be frequency.
If you google each word and their derivatives, here’s the number of entries you get:
irre – 4,790,000 (of which 4,780,000 are also listed on google.de)
irr – 11,600,000
irrsinn – 441,000
irrsinnig – 163,000
wahn – 2,100,000
wahnsinn – 4,290,000
wahnsinnig – 1,660,000
At first glance, this would seem to indicate that wahnsinnig is overwhelmingly more common than irrsinnig, and that would mean we have, in fact, found a “difference” (Not that I would lay claim to any authority here, but this fits my own experience – I feel more comfortable with “wahnsinnig teuer” than with “irrsinnig teuer.” But, I want to stress, that’s little to nothing to go on. And, experience shows, you won’t get a more authoritative answer from German native speakers necessarily, unless you are sure you’re talking not only to a native speaker but to one who can be objective about his/her own language.
And there is an even bigger reason to use caution in assuming you are onto something here, and that is the unreliability of using google frequencies. Whether or not the frequency numbers are telling in this instance (my instincts tell me they probably are), when you type in something for google to count, google doesn’t stop with any single language. If there is a word “irr” in Estonian, for example, those tokens would be counted as well as the German tokens. (I use Estonian as a randomly selected language, incidently. Have no idea what other languages, if any, may have “irr” for a word.) To try to eliminate those, you might be tempted to go to the German google (google.de) and try, and if you do you see 20,000 fewer entries. But remember google.de is not necessarily tapping into German-only sources either!
Sometimes, you can trace the modern day meaning of a word back to its historical origins.
Wahn is a noun meaning “delusion” and
irr(e) is an adjective/adverb meaning “insane”
but that doesn’t help you, since the average person cannot distinguish between what is insane and what is delusional, and even if they could, there is no arguable difference in common usage.
If you’re still determined to find difference, there is much more you can do by pursuing word associations. One could fill pages this way. I’ll just give a couple examples. Wahn not only implies madness, it is the word chosen by the translator of Faulkner’s The Sound and the Fury – Schall und Wahn. We’re getting a bit too far afield here, and ought to stick to irr-sinnig and wahn-sinnig, the words of your original question, but there is always the possibility that the average person’s understanding of word meaning is affected by the range of associated words. Irr may originally have meant insanity, but it also now means “error”, as in irrige Ansicht (misconception) and Irrfahrt (wild-goose chase) and Irrgarten (error-garden, i.e., maze, or labyrinth), and Irrlehre (false doctrine/heresy). Whether that’s “two different words” doesn’t really matter; it’s the association in people’s minds that matters.
So that’s it. It’s entirely possible I am missing something terribly obvious and important here. By all means, if you learn that’s the case, please let me know so I don’t continue in ignorance. But, to answer your question, I would modify what I said originally (“There’s no difference.”) and say “There’s no difference that makes a big difference” except that wahnsinnig is probably used with greater frequency.
Thanks for the ride around the block.