The national flower of Northern Ireland is the shamrock

A three-leaved plant similar to clover. An Irish tale tells of how Patrick used the three-leafed shamrock to explain the Trinity. He used it in his sermons to represent how the Father, the Son, and the Holy Spirit could all exist as separate elements of the same entity. His followers adopted the custom of wearing a shamrock on his feast day.
 http://julieroberts.com.sg/content/index.php