Above, Maeve linked to a Rita Connolly performance of the Shaun Davey "Deer's Cry." Each verse begins with "I Arise Today." Wikipedia says the 1983 Shaun Davey composition uses lyrics based on a translation by Kuno Meyer.

I think this YouTube recording is taken from Shaun Davey's 1983 Pilgrim album, on which Connolly was a soloist.

I had thought the title "The Deer's Cry" came from Davey, but apparently it is a traditional name for the prayers also known as "St. Patrick's Breastplate" and "The Lorica (protection prayer) of St. Patrick."

This Website (click) definitively claims that the hymn was written by Patrick himself in 433. I tend to doubt definitive claims...

The prayer is often called "Faeth Fiada" in Irish, but I don't know that the author actually gave it a title. The Latin word lōrīca originally meant "armor" or "breastplate."

-Joe-