The prayer is part of the Liber Hymnorum, a collection of hymns found in two manuscripts kept in Dublin and published in 1903 in the Thesaurus Paleohibernicus. The document gives this account of how Saint Patrick used this prayer:
“Saint Patrick sang this when an ambush was laid against his coming by Loegaire, that he might not go to Tara to sow the faith. And then it appeared before those lying in ambush that they (Saint Patrick and his monks) were wild deer with a fawn following them.”
The description concludes “fáeth fiada a hainm”, which the Thesaurus Paleohibernicus translates as “Its name is ‘Deer’s Cry’. However, the phrase ‘fáeth fiada’ is used elsewhere in Irish mythology to mean a mist of concealment.