Although irritancy certainly exists as a noun, something that is irritating is normally referred to as an irritant or an irritation — or less commonly and usually with a human agent only, an irritator.
(lifted out of ephemeral comments)
I haven’t found any dictionaries that yet contain irritance, although there do exist published books that happen to use it. This usually means that a word is too new or too rare to bother mentioning, since even catachrestic uses are documented by full dictionaries.
So although anyone would instantly understand what you meant by the word irritance, your computerized spellchecker might not like it and your readers might consider it a non-standard use.
It is important to remember that no dictionary purports to list all words, not even the OED. So just because a dictionary doesn’t list a word does not mean that that word is “not a word”. Absence of evidence never constitutes evidence of absence. There are many reasons why dictionaries leave words out.
You should therefore never conclude that the omission of a dictionary entry for a word somehow “means” that that word is not an “actual” word. As the link to published works shows, there actually are a few people who do — albeit on rather rare occasion — write irritance in published books. A few examples of actual printed use for irritance are:
But it is the interaction, perhaps even mutual irritance, set up between these two worldviews and their increasing lack of it, that motivates many of the striking cultural innovations of the twelfth century.
―The Medieval Greek Romance, by Roderick Beaton, 2012
... from a German perspective about those people who in fact were deported. Survivors of the Nazi regime are thereby labeled as a national irritance and Jews of younger generations become part of the reminder to Germans of a history that they might rather forget.
―“Journalismus als Beruf: Truth and the anti-Semitic journalist stereotype in the writings of Maxim Biller and Rafael Seligmann”, dissertation by Robert Anthony Feldman, 2008
Another feature of lung irritance is the longer term effect, which occurred following exposures to the atmosphere produced by the pyrolysis of flexible polyurethane foam at 600° C, and also by the non-flaming oxidative decomposition of ...
―Advances in Combustion Toxicology by Gordon E. Hartzell , 1988
CTC International undertakes human studies to assess cutaneous irritance, sensitisation potential and the efficacy an acceptability of a wide range of products.
―New Scientist, advertisement, 1984
My personal recommendation would be not to use irritance to mean an irritant, but there is no law forbidding you from doing so.