St Patrick (385 – 461) is recognised as the patron saint of Ireland and is commemorated on 17 March every year.

St Patrick with Shamrock, stained glass window in St. Benin's Church, Kilbennan, County Galway, Ireland - Wikipedia

St Patrick was actually born in the UK but was captured by Irish pirates and taken to Ireland as a boy and sold into slavery. He escaped and came back to the UK, became a priest and returned to Ireland as a missionary.

Although never officially canonized, he was a proclaimed a saint by popular acclaim.

His link to the Shamrock
It is claimed that Patrick used the three-leaved Shamrock, part of the Clover family, to explain the Holy Trinity: the father, son and the Holy Spirit of the Christian church. The three leaves are also used to represent faith, hope and love.

Shamrock leaf courtesy Wikipedia

Apparently, at the time, in pagan Ireland, three was a significant number as the Irish had a number of triple deities which is thought could have aided St Patrick in this teachings. So when he arrived in Ireland in 431, he used the shamrock to teach the Holy Trinity. In pagan Ireland, their attachment to triple deities, could have aided St Patrick in his evangelisation efforts.

The Shamrock
The Shamrock is part of the Clover family but unlike Clover which can have four leaves, Shamrock only has three leaves. It was traditionally used for medicinal purposes.

Shamrock plant courtesy of Wikipedia

According to Wikipedia, ‘there is no one "true" species of shamrock, but that Trifolium dubium (lesser clover) is considered to be the shamrock by roughly half of Irish people, and Trifolium repens (white clover) by another third, with the remaining sixth split between Trifolium pratense (red clover), Medicago lupulina (black medick), Oxalis acetosella (wood sorrel), and various other species of Trifolium and Oxalis.’

‘None of the species in the survey are unique to Ireland, and all are common European species, so there is no botanical basis for the belief that the shamrock is a unique species of plant that only grows in Ireland.’