The trefoil or Shamrock, at one time called the "Seamroy", symbolises the cross and blessed trinity. Before the Christian era it was a sacred plant of the Druids of Ireland because its leaves formed a triad.

The well known legend of the Shamrock connects it definitely to St. Patrick and his teaching.

Preaching in the open air on the doctrine of the trinity, he is said to have illustrated the existence of the Three in One by plucking a shamrock from the grass growing at his feet and showing it to his congregation. The legend of the shamrock is also connected with that of the banishment of the serpent tribe from Ireland by a tradition that snakes are never seen on trefoil and that it is a remedy against the stings of snakes and scorpions.

The trefoil in Arabia is called shamrakh and was sacred in Iran as an emblem of the Persian triads. The trefoil, as noted above, being a sacred plant among the Druids, and three being a mystical number in the Celtic religion as well as all others, it is probable that St. Patrick must have been aware of the significance of his illustration.