This prayer is part of the Liber Hymnorum, an 11th-century collection of hymns found in two manuscripts kept in Dublin

The Liber Hymnorum gives this account of how Saint Patrick used this prayer:

Saint Patrick sang this when an ambush was laid against him and his men by Loegaire son of Niaqll, that he might not go to Tara to sow the faith. And then it appeared before those lying in ambush that they were wild deer with a fawn following them.

The description concludes "fáeth fiada a hainm", which was interpreted as "Deer's Cry" by the medieval editor of the Liber Hymnorum (hence the connection to the deer metamorphosis), but the Old Irish fáeth fiada properly refers to a "mist of concealment". (wikapedia)

sung also at the Inauguration of Michael Higgins, President of Ireland in 2011

https://www.youtube.com/watch?v=miIECtoNQ4M