The Collaborative International Dictionary of English v.0.48:

Irritant \Ir"ri*tant\, a. [LL. irritants, -antis, p. pr. of
   irritare to make null, fr. L. irritus void; pref. ir- not +
   ratus established.] (Scots Law)
   Rendering null and void; conditionally invalidating.
   [1913 Webster]

         The states elected Harry, Duke of Anjou, for their
         king, with this clause irritant; that, if he did
         violate any part of his oath, the people should owe him
         no allegiance.                           --Hayward.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Irritant \Ir"ri*tant\, a. [L. irritans, -antis, p. pr. of
   irritare: cf. F. irritant. See Irritate to excite.]
   Irritating; producing irritation or inflammation.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Irritant \Ir"ri*tant\, n. [Cf. F. irritant.]
   [1913 Webster]
   1. That which irritates or excites.
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   2. (Physiol. & Med.) Any agent by which irritation is
      produced; as, a chemical irritant; a mechanical or
      electrical irritant.
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   3. (Toxicology) A poison that produces inflammation.
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   Counter irritant. See under Counter.

   Pure irritant (Toxicology), a poison that produces
      inflammation without any corrosive action upon the
      tissues.
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WordNet (r) 3.0 (2006):

irritant
    n 1: something that causes irritation and annoyance; "he's a
         thorn in my flesh" [syn: irritant, thorn]