The Collaborative International Dictionary of English v.0.48:

overenthusiastic \overenthusiastic\ adj.
   unduly enthusiastic.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

overenthusiastic
    adj 1: unduly enthusiastic

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "overenthusiastic":
   accident-prone, at fever pitch, bigoted, breakneck, careless,
   delirious, desperate, devil-may-care, extravagant, extreme,
   extremist, fanatic, fanatical, febrile, feverish, frantic,
   frenetic, frenzied, furious, haggard, harum-scarum, hasty,
   headlong, hectic, hotheaded, hurried, hysteric, impetuous,
   infatuated, inordinate, insane, irrational, mad, overanxious,
   overdesirous, overeager, overreligious, overzealous, perfervid,
   precipitant, precipitate, precipitous, rabid, reckless, slap-bang,
   slapdash, ultrazealous, unreasonable, wanton, wild, wild-eyed,
   wild-looking, zealotic