The Barracks Settlement is accessed by either the well from the Dark Alley or the Train.

On the first visit, there won't be any Pirori because of the Mouth Monster so you'll have to encage the latter. That is done by setting up the cage with the Wood Plank, using the Red Eye as bait and luring the Beast there, quickly removing the plank so the cage will close.

The Pirori will then come out from their shelters again.