1755, from irritable + -ity or else from Latin irribilitas.
Entries linking to irritability
irritable adj.
1660s, "susceptible to mental irritation," from French irritable and directly from Latin irritabilis "easily excited," from irritare "excite, provoke" (see irritate). Meaning "responding quickly to a stimulus" is from 1791. Related: Irritably.
-ity
word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).
Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]