From Longman Dictionary of Contemporary Englishldoce_296_ashamrocksham‧rock /ˈʃæmrɒk $ -rɑːk/ noun [countable] 
HBPa small plant with three green leaves on each stem, that is the national symbol of Ireland
HBPa small plant with three green leaves on each stem, that is the national symbol of IrelandExamples from the Corpus
shamrock• This is the second leaf of the shamrock.• They wear the leaves of the shamrock in his honour.• They are worn with their roots intact and if the hydrogel is watered, the shamrock will continue growing.• The shamrock might better be seen as a four-square field, with customers being, as he suggested, the fourth element.• Why is the lovely silver Royal Victorian Order insignia decorated with a rose, thistle, shamrock - and a sunflower?• For similar reasons there was a ban on republicans wearing shamrocks as loyalists considered them provocative.• His patchy mongrel pants like an iris where shamrock fans forget to blur.Origin shamrock (1500-1600) Irish Gaelic seamrog 