Final answer:
Saint Patrick used the shamrock as a national symbol of Ireland to explain the Trinity.
Step-by-step explanation:
The national symbol of Ireland that Saint Patrick used to explain the Trinity is the shamrock. According to legend, Saint Patrick used the three-leafed clover to illustrate the concept of the Holy Trinity, with each leaf representing the Father, Son, and Holy Spirit. The shamrock became closely associated with Saint Patrick and has since become a symbol of Irish identity and culture.