In addition to iron, salt is also used against the water horses. Supernatural creatures hate salt more than someone with high blood pressure, and the capaill are no different. This little bit of myth wouldn't bear mentioning except for one other seemingly unrelated mention of salt.
Finn makes Puck hot chocolate with a special ingredient: Salt. He says, "salt [...] makes cocoa sweeter" (5.33), and even though Puck tastes it and thinks it awesome, she just cannot understand how salt makes anything sweeter. Why is this little scene in the novel? A humorous anecdote from Maggie Stiefvater? Or could a character in the book be the "salt" that makes the world a little sweeter? Both Puck and Sean are a little salty, so maybe they're the spice missing in the other person's life. Or maybe we're overthinking this when we really could use a mug of hot cocoa. Mmm, cocoa.