The Collaborative International Dictionary of English v.0.48:

Irrepressible \Ir`re*press"i*ble\, a.
   Not capable of being repressed, restrained, or controlled;
   as, irrepressible joy; an irrepressible conflict. --W. H.
   Steward.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irrepressible
    adj 1: impossible to repress or control; "an irrepressible
           chatterbox"; "uncontrollable laughter" [syn:
           irrepressible, uncontrollable]

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "irrepressible":
   abandoned, beaming, beyond control, blithe, blithesome, boisterous,
   breachy, bright, bright and sunny, bubbling, buoyant, cheerful,
   cheery, contumacious, defiant, ebullient, effervescent, elated,
   enthusiastic, eupeptic, euphoric, exalted, exhilarated, flushed,
   fractious, genial, glad, gladsome, glowing, go-go, happy, high,
   hopeful, immoderate, in good spirits, in high spirits, incontinent,
   incorrigible, indocile, indomitable, indulgent, insuppressible,
   intemperate, intractable, laughing, lax, licentious, loose,
   nonrestrictive, obstreperous, of good cheer, optimistic,
   out of control, out of hand, permissive, pleasant, radiant,
   rampant, recalcitrant, refractory, reinless, resistant, resisting,
   restive, rhapsodical, riant, riotous, rosy, sanguine, sanguineous,
   shrewish, smiling, sparkling, sunny, unbiddable, unbridled,
   unchecked, uncoerced, uncompelled, unconstrained, uncontrollable,
   uncontrolled, uncurbed, unforced, ungovernable, ungoverned,
   uninhibited, unmalleable, unmanageable, unmastered, unmeasured,
   unmoldable, unmuzzled, unreined, unrepressed, unreserved,
   unrestrained, unrestrictive, unruly, unstoppable, unsubdued,
   unsubmissive, unsuppressed, wanton, wild, winsome