The Collaborative International Dictionary of English v.0.48:

Shamrock \Sham"rock\, n. [L. seamrog, seamar, trefoil, white
   clover, white honeysuckle; akin to Gael. seamrag.] (Bot.)
   A trifoliate plant used as a national emblem by the Irish.
   The legend is that St. Patrick once plucked a leaf of it for
   use in illustrating the doctrine of the trinity.
   [1913 Webster]

   Note: The original plant was probably a kind of wood sorrel
         (Oxalis Acetocella); but now the name is given to the
         white clover (Trifolium repens), and the black medic
         (Medicago lupulina).
         [1913 Webster]

WordNet (r) 3.0 (2006):

shamrock
    n 1: creeping European clover having white to pink flowers and
         bright green leaves; naturalized in United States; widely
         grown for forage [syn: white clover, dutch clover,
         shamrock, Trifolium repens]
    2: Eurasian plant with heart-shaped trifoliate leaves and white
       purple-veined flowers [syn: common wood sorrel, cuckoo
       bread, shamrock, Oxalis acetosella]
    3: clover native to Ireland with yellowish flowers; often
       considered the true or original shamrock [syn: hop clover,
       shamrock, lesser yellow trefoil, Trifolium dubium]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "shamrock":
   armory, badge, badge of office, badges, baton, blazonry, brassard,
   button, cap and gown, chain, chain of office, class ring, clover,
   cockade, collar, cross, decoration, deuce-ace, dress, eagle,
   emblems, ensigns, fasces, figurehead, fleur-de-lis,
   hammer and sickle, heraldry, insignia, lapel pin, leash, livery,
   mace, mantle, markings, medal, mortarboard, old school tie, pin,
   regalia, ring, rose, school ring, set of three, sigillography,
   skull and crossbones, sphragistics, staff, swastika, tartan,
   tercet, ternary, ternion, terzetto, thistle, three, threesome, tie,
   tierce, trefoil, trey, triad, trialogue, triangle, tricorn,
   trident, triennium, trihedron, trilogy, trimester, trine, trinity,
   trinomial, trio, triphthong, triple crown, triple threat, triplet,
   triplopy, tripod, triptych, trireme, triseme, triskelion, trisul,
   triumvirate, triunity, trivet, troika, uniform, verge, wand



U.S. Gazetteer Places (2000):

Shamrock, OK -- U.S. town in Oklahoma
   Population (2000):    125
   Housing Units (2000): 70
   Land area (2000):     0.329684 sq. miles (0.853877 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.329684 sq. miles (0.853877 sq. km)
   FIPS code:            66600
   Located within:       Oklahoma (OK), FIPS 40
   Location:             35.910737 N, 96.580131 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Shamrock, OK
    Shamrock


U.S. Gazetteer Places (2000):

Shamrock, TX -- U.S. city in Texas
   Population (2000):    2029
   Housing Units (2000): 1072
   Land area (2000):     2.070994 sq. miles (5.363849 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    2.070994 sq. miles (5.363849 sq. km)
   FIPS code:            67160
   Located within:       Texas (TX), FIPS 48
   Location:             35.217116 N, 100.247171 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Shamrock, TX
    Shamrock