Traditionally, shamrock is said to have been used by Saint Patrick to illustrate the Christian doctrine of the Trinity when Christianising Ireland in the 5th century. However, this is described by the Oxford English Dictionary a "a late tradition", first recorded in 1726, and is probably false. Nonetheless, since the 18th century, shamrock has been used as a symbol of Ireland in a similar way to how a rose is used for England, thistle for Scotland and leek for Wales.
Shamrock commonly appears as part of the emblem of sporting and official organisations representing both the whole of Ireland (such as the Irish Rugby Football Union or Tourism Ireland) as well as organisations specific to the Republic of Ireland (such as IDA Ireland) and Northern Ireland (such as Police Service of Northern Ireland). Shamrock is also used in emblems of UK organisations with an association with Ireland, such as the Irish Guards. Outside Ireland, various organisations, businesses and places use the symbol to advertise a connection with the island. For example, basketball team, Boston Celtics, in the USA incorporate the shamrock in their logo and the US cereal, Lucky Charms, uses it on the product's mascot and as a shape in the cereal itself.
The shamrock has been registered as a trademark by the Government of Ireland. Traditionally in Ireland, and in many places throughout the world, the shamrock is worn on the lapel on St. Patrick's Day.
In the early 1980s, Ireland defended its right to use the shamrock as its national symbol in a German trademark case, which included high-level representation from taoiseach Charles Haughey. Having originally lost, Ireland won on appeal to the German Supreme Court in 1985.