The Deer Cry, better known in English as Saint Patrick's Breastplate, is a lorica, a protective prayer that names the forces invoked and the dangers guarded against, a recognised genre in early Irish and British Christianity.
Its structure is invocation by binding: the speaker binds to himself the strength of the Trinity, then the power of Christ's incarnation, baptism, crucifixion, resurrection and ascension, then the virtues of angels, patriarchs, prophets, apostles and confessors, then the elements themselves, the sun, fire, lightning, wind, sea, earth and rock. It then lists what it defends against, and the list is specific: the snares of demons, the spells of women, smiths and druids.
The most quoted section is the sequence beginning Christ with me, Christ before me, Christ behind me, which has entered general devotional use well outside Ireland.
The attribution to Patrick is traditional and is not defensible on linguistic grounds. Patrick was a fifth-century Briton who wrote in Latin, and two Latin works of his survive, the Confessio and the Letter to Coroticus, whose style is plain and rather anxious. The Deer Cry is in Old Irish, and the language belongs to roughly the eighth century. It is preserved in the Liber Hymnorum with a preface supplying the legend: that Patrick and his companions sang it while passing an ambush and appeared to their attackers as deer, which is where the name comes from.