The Shamrock symbolizes the cross as well as the Blessed Trinity.
Prior to the arrival of St. Patrick in Ireland it was considered a sacred plant of the Druids.
St. Patrick is said to have used the shamrock in his preaching to illustrate the doctrine of the Trinity, with the three leaves representing the Father, the Son and the Holy Spirit, thus making up the One Triune God.
May the luck of the Irish lead to happiest heights
And the highway you travel be lined with green lights.