The Collaborative International Dictionary of English v.0.48:

Irritable \Ir"ri*ta*ble\, a. [L. irritabilis: cf. F. irritable.
   See Irritate.]
   [1913 Webster]
   1. Capable of being irritated.
      [1913 Webster]

   2. Very susceptible of anger or passion; easily inflamed or
      exasperated; as, an irritable temper.
      [1913 Webster]

            Vicious, old, and irritable.          --Tennyson.
      [1913 Webster]

   3. (Physiol.) Endowed with irritability; susceptible of
      irritation; capable of being excited to action by the
      application of certain stimuli.
      [1913 Webster]

   4. (Med.) Susceptible of irritation; unduly sensitive to
      irritants or stimuli. See Irritation, n., 3.

   Syn: Excitable; irascible; touchy; fretful; peevish.
        [1913 Webster]

WordNet (r) 3.0 (2006):

irritable
    adj 1: easily irritated or annoyed; "an incorrigibly fractious
           young man"; "not the least nettlesome of his countrymen"
           [syn: cranky, fractious, irritable, nettlesome,
           peevish, peckish, pettish, petulant, scratchy,
           testy, tetchy, techy]
    2: abnormally sensitive to a stimulus
    3: capable of responding to stimuli [syn: excitable,
       irritable]