Another very Irish thing, the Blarney Stone, is also believed to be originally from Scotland.
The shamrock was used by St. Patrick to explain the Trinity - the Father, Son and Holy Spirit.
Trinity symbols are used in many different cultures and symbolise unity in diversity.
Celtic Irish - three Bridget's
Egypt - Osiris, Isis and Horus
Roman - Jupiter, Neptune, Pluto [sky, ocean and underworld]
Patrick was born as Maewyn Succat (Succat sounds like sucata, Portuguese for scrap metal). Patrick comes from the Roman patrician, or noble and a strange choice for one so humble.