I think the florists have convinced Americans that the Oxalis (wood sorrel) is the true shamrock because it is larger and showier, South American and South African species especially. The original was probably the white clover, Trifolium repens, but there is no way of proving that.

Shamrock comes from the Irish seamróg (Gaelic seamrig), diminutive of seamar, or CLOVER. These are Anglicized spellings, someone will probably post the Gaelic.