The shamrock, a symbol of Ireland and a registered trademark of the Republic of Ireland, is a three-leafed old white clover, sometimes of the variety Trifolium repens (a white clover, known in Irish as seamair bhán) but today usually Trifolium dubium (a lesser clover, Irish: seamair bhuí).
The diminutive version of the Irish word for "clover" ("seamair") is "seamróg", which was anglicised as "shamrock", representing a close approximation of the original Irish pronunciation. However, other three-leafed plants — such as black medic (Medicago lupulina), red clover (Trifolium pratense), and common wood sorrel (genus Oxalis) — are sometimes designated as shamrocks. 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 Saint Patrick's Day. Shamrocks are said to bring good luck.
The shamrock is also informally used as an emblem for sports teams, state organisations, and troops abroad from Ireland: the IRFU, Cliftonville F.C. Shamrock Rovers F.C., Aer Lingus, IDA Ireland, University College Dublin, University of Notre Dame, the Northern Ireland Tourist Board and Fáilte Ireland use it as part of their identity, but it should be noted that according to the Irish Constitution, the Gaelic or Celtic harp (often called "Brian Boru's Harp"), is the primary symbol for Ireland, appearing on postage stamps, government insignia, armed forces insignia and the coat of arms of the President of Ireland. It is registered with the World Intellectual Property Organization as a symbol of Ireland. According to what the Oxford English Dictionary calls "a late tradition" (first recorded in 1726), the plant was used by Saint Patrick to illustrate the doctrine of the Trinity. The posthumous timing of this legend (coming some 1200 years after his death), and the lack of supporting evidence found in St. Patrick's writings have caused some to question its authenticity.. In pre-Christian Ireland long before the arrival of Patrick, the shamrock was the principal symbol of the Irish Triple Goddess.
The shamrock is featured on the passport stamp of Montserrat, many of whose citizens are of Irish descent. In addition, the shamrock is frequently used as a name and symbol for Irish pubs throughout the world.
The flag of the city of Montréal, Canada has a shamrock in the lower right quadrant. The shamrock represents the Irish population, one of the four major ethnic groups that made up the population of the city in the 19th century when the arms were designed.
The coat of arms on the flag of the Royal Ulster Constabulary George Cross Foundation was cradled in a wreath of shamrock.
The Erin Go Bragh flag uses an angelic Cláirseach, a medieval Irish harp, cradled in a wreath of clover. A flag strongly symbolic of Irish nationalism, it is often seen on Saint Patrick's Day, usually displayed during the parades.