Source : Webster's Revised Unabridged Dictionary (1913)

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

            Cold maketh the spirits vigorous and irritateth
            them.                                 --Bacon.

   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.

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

   3. (Physiol.) To produce irritation in; to stimulate; to
      cause to contract. See {Irritation}, n., 2.

   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.

Source : WordNet®

irritating
     adj 1: causing irritation or annoyance; "tapping an annoying rhythm
            on his glass with his fork"; "aircraft noise is
            particularly bothersome near the airport"; "found it
            galling to have to ask permission"; "an irritating
            delay"; "nettlesome paperwork"; "a pesky mosquito";
            "swarms of pestering gnats"; "a plaguey newfangled
            safety catch"; "a teasing and persistent thought
            annoyed him"; "a vexatious child"; "it is vexing to
            have to admit you are wrong" [syn: {annoying}, {bothersome},
             {galling}, {nettlesome}, {pesky}, {pestering}, {pestiferous},
             {plaguy}, {plaguey}, {teasing}, {vexatious}, {vexing}]
     2: (used of physical stimuli) serving to stimulate or excite;
        "an irritative agent" [syn: {irritative}]
     3: causing physical discomfort; "bites of black flies are more
        than irritating; they can be very painful" [syn: {painful}]