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]