The Collaborative International Dictionary of English v.0.48:

Irritate \Ir"ri*tate\, a.
   Excited; heightened. [Obs.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Irritate \Ir"ri*tate\, v. t. [See 1 st Irritant.]
   To render null and void. [R.] --Abp. Bramhall.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Irritate \Ir"ri*tate\, v. t. [imp. & p. p. Irritated; p. pr. &
   vb. n. Irritating.] [L. irritatus, p. p. of irritare. Of
   doubtful origin.]
   [1913 Webster]
   1. To increase the action or violence of; to heighten
      excitement in; to intensify; to stimulate.
      [1913 Webster]

            Cold maketh the spirits vigorous and irritateth
            them.                                 --Bacon.
      [1913 Webster]

   2. To excite anger or displeasure in; to provoke; to tease;
      to exasperate; to annoy; to vex; as, the insolence of a
      tyrant irritates his subjects.
      [1913 Webster]

            Dismiss the man, nor irritate the god:
            Prevent the rage of him who reigns above. --Pope.
      [1913 Webster]

   3. (Physiol.) To produce irritation in; to stimulate; to
      cause to contract. See Irritation, n., 2.
      [1913 Webster]

   4. (Med.) To make morbidly excitable, or oversensitive; to
      fret; as, the skin is irritated by friction; to irritate a
      wound by a coarse bandage.

   Syn: To fret; inflame; excite; provoke; tease; vex;
        exasperate; anger; incense; enrage.

   Usage: To Irritate, Provoke, Exasperate. These words
          express different stages of excited or angry feeling.
          Irritate denotes an excitement of quick and slightly
          angry feeling which is only momentary; as, irritated
          by a hasty remark. To provoke implies the awakening of
          some open expression of decided anger; as, a provoking
          insult. Exasperate denotes a provoking of anger at
          something unendurable. Whatever comes across our
          feelings irritates; whatever excites anger provokes;
          whatever raises anger to a high point exasperates.
          "Susceptible and nervous people are most easily
          irritated; proud people are quickly provoked; hot and
          fiery people are soonest exasperated." --Crabb.
          [1913 Webster]

WordNet (r) 3.0 (2006):

irritate
    v 1: cause annoyance in; disturb, especially by minor
         irritations; "Mosquitoes buzzing in my ear really bothers
         me"; "It irritates me that she never closes the door after
         she leaves" [syn: annoy, rag, get to, bother, get
         at, irritate, rile, nark, nettle, gravel, vex,
         chafe, devil]
    2: excite to an abnormal condition, or chafe or inflame;
       "Aspirin irritates my stomach" [ant: soothe]
    3: excite to some characteristic action or condition, such as
       motion, contraction, or nervous impulse, by the application
       of a stimulus; "irritate the glands of a leaf"

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "irritate":
   abrade, afflict, affront, agent provocateur, aggravate, agonize,
   ail, alienate, amplify, anger, annoy, arouse, augment, bite,
   bother, bristle, bug, build up, burn, burn up, chafe, come between,
   convulse, crucify, cut, damage, deepen, deteriorate, dilapidate,
   disaffect, distress, disunite, divide, embitter, endamage, enhance,
   enlarge, enrage, estrange, exacerbate, exasperate, excruciate,
   exercise, fan the flame, fester, fluster, fret, gall, get,
   give pain, gnaw, grate, grate on, gravel, grind, gripe, grit,
   harass, harm, harrow, harry, hassle, heat up, hector, heighten,
   hot up, huff, hurt, impair, incense, increase, inflame,
   inflict pain, injure, intensify, irk, jangle the nerves, jar on,
   kill by inches, lacerate, light the fuse, magnify, make acute,
   make trouble, make worse, martyr, martyrize, miff, nag, needle,
   nettle, nip, offend, pain, peeve, pester, pick at, pick on, pierce,
   pinch, pique, pit against, plague, prick, prolong the agony,
   provoke, put back, put on edge, put out, put to torture, rack,
   rankle, rasp, rile, roil, rub, ruffle, separate, set against,
   set at odds, set at variance, set on, set on edge, set up, sharpen,
   sic on, sour, sow dissension, stab, sting, stir the blood, stir up,
   stir up trouble, torment, torture, trouble, try, tweak, twist, vex,
   weaken, work up, worry, worsen, wound, wring