The image below this post, of a cross with the text of the Breastplate of St. Patrick, I found on the Brothers of St. Patrick Monastery’s website. The text of the prayer can be found all over, such as on this page.
I was curious about the story behind this prayer, so I did some research.
From what I gather, after Pope Celestine I sent Patrick to northern and western Ireland to spread the gospel there–where it had never been before–, Patrick knew that in order to convert the people, he had to start with the head.
The people of Ireland were, of course, living in tribes. It was the Druids who saw to it that the people stayed away from what they thought were false religions, so naturally Patrick knew as he was making his way that he was going to have to deal with them. They were known to be a violent group of people, so Patrick was naturally concerned.
But this was no deterrent. Onward Patrick went to the chief of the most powerful clan, the Tara, knowing that if he could convert this chief, others would follow. But he had to go past the Druids.
Story goes that as he was on his way to the chief’s court, Patrick said this prayer together with the other men who were making the journey with him. It did the trick: the Druids did not see Patrick and his men pass, and in fact mistook them as deer!
Hence, the city is now called “Deer City.”
As soon as Patrick got to the chief, he took quite a liking to Patrick. After his conversion, Christianity took root in Ireland.
Of course, the Druids, upon learning that he had made it past them, continued to plot against him, but Patrick simply prayed this prayer with complete trust in God.