sham-rock
noun

Any of several plants, such as a clover or wood sorrel, having compound leaves with three small leaflets, considered the national emblem of Ireland.

Irish Gaelic seamróg, diminutive of seamar, clover, from Middle Irish semar

Source: The American Heritage® Dictionary of the English Language, Fourth Edition


And for all you passionate botanists, here's what Wordnet has to say:

sham-rock

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, Trifolium repens]
2: Eurasian plant with heart-shaped trifoliate leaves and white pink- or purple-veined flowers [syn: common wood sorrel, cuckoo bread, Oxalis acetosella]
3: clover native to Ireland with yellowish flowers; often considered the true or original shamrock [syn: hop clover, lesser yellow trefoil, Trifolium dubium]

Source: WordNet ® 2.0, © 2003 Princeton University

Geographical note:
There are 2 towns in the US named Shamrock, one in Oklahoma, the other in Texas.

Cultural note:
It is believed that if you find a 4-leaf clover you will have great luck :)