The Collaborative International Dictionary of English v.0.48:

Irritation \Ir`ri*ta"tion\, n. [L. irritatio: cf. F.
   irritation.]
   [1913 Webster]
   1. The act of irritating, or exciting, or the state of being
      irritated; excitement; stimulation, usually of an undue
      and uncomfortable kind; especially, excitement of anger or
      passion; provocation; annoyance; anger.
      [1913 Webster]

            The whole body of the arts and sciences composes one
            vast machinery for the irritation and development of
            the human intellect.                  --De Quincey.
      [1913 Webster]

   2. (Physiol.) The act of exciting, or the condition of being
      excited to action, by stimulation; -- as, the condition of
      an organ of sense, when its nerve is affected by some
      external body; esp., the act of exciting muscle fibers to
      contraction, by artificial stimulation; as, the irritation
      of a motor nerve by electricity; also, the condition of a
      muscle and nerve, under such stimulation.
      [1913 Webster]

   3. (Med.) A condition of morbid excitability or
      oversensitiveness of an organ or part of the body; a state
      in which the application of ordinary stimuli produces pain
      or excessive or vitiated action.
      [1913 Webster]

WordNet (r) 3.0 (2006):

irritation
    n 1: the psychological state of being irritated or annoyed [syn:
         irritation, annoyance, vexation, botheration]
    2: a sudden outburst of anger; "his temper sparked like damp
       firewood" [syn: pique, temper, irritation]
    3: (pathology) abnormal sensitivity to stimulation; "any food
       produced irritation of the stomach"
    4: the neural or electrical arousal of an organ or muscle or
       gland [syn: excitation, innervation, irritation]
    5: an uncomfortable feeling of mental painfulness or distress
       [syn: discomfort, soreness, irritation]
    6: unfriendly behavior that causes anger or resentment [syn:
       aggravation, irritation, provocation]
    7: the act of troubling or annoying someone [syn: annoyance,
       annoying, irritation, vexation]

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "irritation":
   adverse circumstances, adversity, affliction, aggravation,
   agitation, algesia, amplification, animation, annoyance, arousal,
   arousing, augmentation, blight, bummer, care, chafe,
   contentiousness, cross, curse, deepening, deliberate aggravation,
   deterioration, difficulties, difficulty, disapprobation,
   disapproval, discontent, displeasure, dissatisfaction, downer,
   electrification, embittering, embitterment, enhancement,
   enlargement, exacerbation, exasperation, excitation, excitement,
   exhilaration, festering, firing, fomentation, fret, gall,
   galvanization, hard knocks, hard life, hard lot, hardcase,
   hardship, heightening, incitation, incitement, increase,
   inflammation, infuriation, instigation, intensification, irritant,
   lathering up, magnification, pep rally, pep talk, perturbation,
   plight, predicament, pressure, provocation, rabble-rousing,
   rankling, resentfulness, resentment, rigor, sea of troubles,
   sensitiveness, sharpening, sore, sore spot, soreness, souring,
   steaming up, stimulation, stimulus, stirring, stirring up,
   stirring-up, stress, stress of life, tenderness, trial,
   tribulation, trouble, troubles, vale of tears, vexation,
   vicissitude, whipping up, working up, worsening