In the Elvish language from the universe or JRR Tolkien, 'Ithil', or 'Isil', translates to 'Moon'. The word 'Boreal' is a word used to describe a region to the far North, often accompanied by a very cold or frigid climate. The southern equivalent of boreal is austral.
""Ir" in Elvish means "when" and is specifically used in statements of time, rather than questions. So, a loose translation or Irithyll could be "The Time of the Moon" or, less poetically, "Night". Perhaps the name references the moon being perpetually risen. Or it could just be a bastardization and mean "Moon". In layman's terms, one could translate 'Irithyll of the Boreal Valley' to 'Moonlit Frigid Valley', 'Moonlight of the Arctic Valley', or something along those lines. A more accurate translation of 'Irithyll of the Boreal Valley' could be "Time of the Moon in the Northern/Arctic Valley".
The varied areas in the game aren't always naturally tied to each other; they may come from different time periods. The Lords of Cinder were revived by the bell when the Linking of the Flame was in danger and abandoned their thrones and returned to their original "churning" lands. But, these lands may actually come from the past as the Lords of Cinder (Aldrich, Ludleth, the Abyss Watchers and Yhorm) each Linked the Flame in a different cycle and the lands they hail from should have long disappeared, and can appear again in Lothric only because space and time are "convoluted". Irithyll looks like a society built around the ruins of Anor Londo in an age when they were still in place, and so were its last occupants (Gwyndolin and the Darkmoon Blades), until the Gods were ultimately betrayed by Sulyvahn and devoured by Aldrich.
Small silverwork doll depicting a young squire.
In the legendary old city of Irithyll situated in the Boreal Valley, the Pontiff Sulyvahn gave this doll to valued subjects, so that they might use it to cross the barrier when they return home.
Listen carefully, and you can hear it say, "Wherever you go, the moon still sets in Irithyll. Wherever you may be, Irithyll is your home."
Outrider Knight Armor
Armor of an Irithyll outrider knight. Enveloped in a dimly cool air.
The knights were given the eyes of the Pontiff, but the eyes transformed them into savage, raving warriors who only knew how to serve as mindless guards.
Irithyll Straight Sword
Straight sword bestowed upon the Outrider Knights of the Boreal Valley. This weapon is enshrouded in frost, and causes frostbite.
Every Outrider Knight one day devolves into a beast, constantly hounded by Pontiff Sulyvahn's black eyes. Irithyll Rapier
Thrusting sword bestowed upon the Outrider Knights of the Boreal Valley. This weapon is shrouded in frost, and causes frostbite.
Every Outrider Knight one day devolves into a beast, constantly hounded by Pontiff Sulyvahn's black eyes.