Faeth Fiada: The Cloak of Invisibility
A magical ability of particular renown was the faeth fiada, or “deer's cry,” the ability to cloak oneself in mist or to appear invisible. This particular talent of the Tuatha Dé Danann, a gift from the sea god Manannan, was later used to cloak Ireland in a magical mist to prevent the invading Milesians from returning to the isle. It is the faeth fiada that prevents the Sidhe folk from being seen today.
Various other mythological characters possessed the ability, including Diarmuid, the nephew of Fionn. Eventually, this shapeshifting talent passed to St. Patrick, who used an incantation called the “Deer's Cry,” which gave him the appearance of a fawn to his enemies.
The incantation, spoken today as a prayer of protection, is called the Lorica, or “Breastplate of Patrick.” It reads in part:
May Christ shield me today
Protective prayers of the sort became increasingly popular until it was quite common practice to tuck a copy of one onto one's person before traveling. Medieval knights were known to have Patrick's poem inscribed inside their own breastplates.
Although much of the text of the Lorica is explicitly Christian, a portion of the prayer betrays its ancient origin:
I arise today through the strength of Heaven
The verse is clearly derived from a bardic formula and recalls the ancient invocations of Amergin to the elements. Although ostensibly Christian, the prayer makes no invocation to the heavenly realm, preferring the immediacy of the earthly sphere. There is no way to know whether the prayer was Christianized to disguise its origin or if its popularity necessitated an acceptible alternative.