The Collaborative International Dictionary of English v.0.48:

Irritancy \Ir"ri*tan*cy\, n. [From 1st Irritant.] (Scots Law)
   The state or quality of being null and void; invalidity;
   forfeiture. --Burrill.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Irritancy \Ir"ri*tan*cy\, n. [From 2d Irritant.]
   The state or quality of being irritant or irritating.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

IRRITANCY. In Scotland, it is the happening of a condition or event by which
a charter, contract or other deed, to which a clause irritant is annexed,
becomes void. Ersk. Inst. B. 2, t. 5, n. 25. Irritancy is a kind of
forfeiture. It is legal or conventional. Burt. Man. P. R. 29 8.