There's a lot going on here!

Verses 2, 3, and 5 are the same, music-wise. Verse 1 is just the first half of that melody.

Verse 4 is very different. It's written in harmony when the others are in unison, and it's in G major where the others are in G minor. (This is called the "parallel minor," because they're both in G, but different Gs.) In fact, it's so different that it's actually listed as a second hymn tune--"Deirdre," where the others are "St. Patrick's Breastplate." There are only a couple examples of this phenomenon, so of course it gets a special tag.

(They're both folk songs, so for the music attribution, it's "Verses 1-3 and 5: Irish melody. Verse 4: also an Irish melody, just a different one.)

What about the text? This is "attributed" (read: it's folklore so we don't know) to St. Patrick, hence the name. Yes, that one--snakes, Irish pride, etc. Also a writer, apparently. The original Irish text, whoever wrote it, is longer and more elaborate and calls on Christ's help in a variety of situations: see this translation by James Clarence Mangan. The verse of that one that begins "At Tara in this fateful hour..." corresponds to our verse 3, but that translation is more famous because it shows up (repeatedly) in "A Swiftly Tilting Planet," one of the sequels to "A Wrinkle in Time" by Madeleine L'Engle. I read that well before I came across this hymn (we hardly ever sing it), and then it was like "ohhhhhh."

Here's yet another loose adaptation my choir did, you can sort of hear the melody from here show up in the piano accompaniment.