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]

Moby Thesaurus II by Grady Ward, 1.0:

122 Moby Thesaurus words for "irritable":
   aggressive, agitable, all nerves, allergic, anaphylactic,
   apprehensive, bad-tempered, bearish, bellicose, belligerent,
   bickering, bitchy, cankered, cantankerous, choleric, churlish,
   combative, crabbed, crabby, cranky, cross, cross-grained,
   crotchety, crusty, cussed, delicate, disagreeable, disputatious,
   divisive, dyspeptic, edgy, emotional, emotionally unstable,
   empathetic, empathic, eristic, eruptive, excitable, explosive,
   factional, factious, fearful, feisty, fractious, fretful,
   frightened, goosy, grouchy, gruff, high-mettled, high-spirited,
   high-strung, highly emotional, huffish, huffy, hyperesthetic,
   hyperpathic, hypersensitive, ill-humored, ill-tempered, impatient,
   inflammable, irascible, itchy, litigious, mean, mettlesome, moody,
   nerves on edge, nervous, nervy, on edge, ornery, overrefined,
   oversensible, oversensitive, overstrung, overtender, panicky,
   partisan, passible, peevish, perturbable, perverse, pettish,
   petulant, pindling, polarizing, polemic, prickly, pugnacious,
   quarrelsome, querulous, raspy, refined, responsive, sensitive,
   short-tempered, shrewish, skittish, snappish, snappy, spiteful,
   spleeny, splenetic, startlish, supersensitive, sympathetic,
   tactful, temperamental, tender, testy, tetchy, thin-skinned,
   ticklish, touchy, ugly, uneasy, volcanic, waspish, whiny,
   wrangling