The shamrock, an unofficial symbol of Ireland, is a three-leafed old white clover, sometimes (rarely nowadays) Trifolium repens (white clover, known in Irish Gaelic as seamair bhán) but more usually today Trifolium dubium (lesser clover, Irish Gaelic: seamair bhuí). The diminutive version of the Gaelic word for "clover" ("seamair") is "seamaróg", which was anglicised as "shamrock", representing a close approximation of the original Gaelic pronunciation. The shamrock was traditionally used for its medical properties and was a popular motif in Victorian times. It is also a common way to represent St. Patrick's Day, a holiday celebrated on March 17. Shamrocks are said to bring good luck.

[This is an excerpt from a Wikipedia article about the shamrock.]