A shamrock is a young sprig, used as a symbol of Ireland. Saint Patrick, Ireland's patron saint, is said to have used it as a metaphor for the Christian Holy Trinity. The name shamrock comes from Irish seamr?g, which is the diminutive of the Irish word seamair ?g and means simply "young clover".

Shamrock usually refers to either the species Trifolium dubium (lesser clover, Irish: seamair bhu?) or Trifolium repens (white clover, Irish: seamair bh?n). However, other three-leaved plants—such as Medicago lupulina, Trifolium pratense, and Oxalis acetosella—are sometimes called shamrocks. The shamrock was traditionally used for its medicinal properties and was a popular motif in Victorian times.

The word shamrock derives from seamair ?g or young clover, and references to semair or clover appear in early Irish literature, generally as a description of a flowering clovered plain. For example, in the series of medieval metrical poems about various Irish places called the Metrical Dindshenchus, a poem about Tailtiu or Teltown in Co. Meath describes it as a plain blossoming with flowering clover (mag scothach scothshemrach). Similarly, another story tells of how St. Brigid decided to stay in Co. Kildare when she saw the delightful plain covered in clover blossom (scoth-shemrach). However, the literature in Irish makes no distinction between clover and shamrock, and it is only in English that shamrock emerges as a distinct word.

The first mention of shamrock in the English language occurs in 1571 in the work of the English Elizabethan scholar Edmund Campion. In his work Boke of the Histories of Irelande, Campion describes the habits of the "wild Irish" and states that the Irish ate shamrock: "Shamrotes, watercresses, rootes, and other herbes they feed upon". The statement that the Irish ate shamrock was widely repeated in later works and seems to be a confusion with the Irish word seams?g or wood sorrel (Oxalis). There is no evidence from any Irish source that the Irish ate clover, but there is evidence that the Irish ate wood sorrel. For example, in the medieval Irish work Buile Shuibhne (The Frenzy of Sweeney), the king Sweeney, who has gone mad and is living in the woods as a hermit, lists wood sorrel among the plants he feeds upon.

Traditionally, shamrock is said to have been used by Saint Patrick to illustrate the Christian doctrine of the Holy Trinity when Christianising Ireland in the 5th century. The first evidence of a link between St Patrick and the shamrock appears in 1675 on the St Patrick's Coppers or Halpennies. These appear to show a figure of St Patrick preaching to a crowd while holding a shamrock, presumably to explain the doctrine of the Holy Trinity. In pagan Ireland, three was a significant number and the Irish had many triple deities, which could have aided St Patrick in his evangelisation efforts. Patricia Monaghan states that "There is no evidence that the clover or wood sorrel (both of which are called shamrocks) were sacred to the Celts". However, Jack Santino speculates that "The shamrock was probably associated with the earth and assumed by the druids to be symbolic of the regenerative powers of nature ... Nevertheless, the shamrock, whatever its history as a folk symbol, today has its meaning in a Christian context. Pictures of Saint Patrick depict him driving the snakes out of Ireland with a cross in one hand and a sprig of shamrocks in the other." Roger Homan writes, "We can perhaps see St Patrick drawing upon the visual concept of the triskele when he uses the shamrock to explain the Trinity". Why the Celts to whom St Patrick was preaching would have needed an explanation of the concept of a triple deity is not clear (two separate triple goddesses are known to have been worshipped in pagan Ireland).

As St. Patrick is Ireland's patron saint, shamrock has been used as a symbol of Ireland since the 18th century, in a similar way to how a rose is used for England, thistle for Scotland and daffodil for Wales. The shamrock first began to change from a symbol purely associated with St. Patrick to an Irish national symbol when it was taken up as an emblem by rival militias, during the turbulent politics of the late eighteenth century. On one side were the Volunteers (also known as the Irish Volunteers), who were local militias in late 18th century Ireland, raised to defend Ireland from the threat of French and Spanish invasion when regular British soldiers were withdrawn from Ireland to fight during the American Revolutionary War. On the other side were revolutionary nationalist groups, such as the United Irishmen.

Among the Volunteers, examples of the use of the shamrock include its appearance on the guidon of the Royal Glin Hussars formed in July 1779 by the Knight of Glin, and its appearance on the flags of the Limerick Volunteers, the Castle Ray Fencibles and the Braid Volunteers. The United Irishmen adopted green as their revolutionary colour and wore green uniforms or ribbons in their hats, and the green concerned was often associated with the shamrock. The song The Wearing of the Green commemorated their exploits and various versions exist which mention the shamrock. The Erin go bragh flag was used as their standard and was often depicted accompanied by shamrocks, and in 1799 a revolutionary journal entitled The Shamroc briefly appeared in which the aims of the rebellion were supported.

Throughout the nineteenth and twentieth centuries, the shamrock continued to appear in a variety of settings. For example, the shamrock appeared on many buildings in Ireland as a decorative motif, such as on the facade of the Kildare Street Club building in Dublin, St. Patrick's Cathedral, Armagh, and the Harp and Lion Bar in Listowel, Co. Kerry. It also appears on street furniture, such as old lamp standards like those in Mountjoy Square in Dublin, and on monuments like the Parnell Monument, and the O'Connell Monument, both in O'Connell Street, Dublin. Shamrocks also appeared on decorative items such as glass, china, jewellery, poplin and Irish lace. Belleek Pottery in Co. Fermanagh, for example regularly features shamrock motifs.

The shamrock also appears in the emblems of a wide range of voluntary and non-state organisations in Ireland, such as the Irish Farmers Association, the Boy Scouts of Ireland association, Scouting Ireland Irish Girl Guides, and the Irish Kidney Donors Association., In addition many sporting organisations representing Ireland use the shamrock in their logos and emblems. Examples include the Irish Football Association (Northern Ireland), Irish Rugby Football Union, Swim Ireland, Cricket Ireland, and the Olympic Council of Ireland. A sprig of shamrock represents the Lough Derg Yacht Club Tipperary, (est. 1836). The shamrock is the official emblem of Irish football club Shamrock Rovers.