The shamrock is a three-leaved young white clover, sometimes (rarely nowadays) Trifolium repens (white clover, seamair bhan) but more usually today Trifolium dubium (lesser clover, seamair bhui). The shamrock was traditionally used as for medical properties and was a popular motif in Victoria times.
The Shamrock is also used as a badge for sports teams, state organisations and troops abroad from Ireland - Aer Lingus, IDA Ireland and University College Dublin use is as part of their identity. It is registered with the World Intellectual Property Organisation 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. It has subsequently become an emblem of Ireland, but it is not the official one, which is the harp. The English word comes from seamróg, an Irish word meaning young clover.
A detailed history, including discussion of the identity of shamrock, by E. C. Nelson (1991) Shamrock, botany and history of an Irish myth ISBN0863141994, is still in print and available direct from the author.