Masked villagers burying "witness mirrors" during the Festival of Reunion

The Festival of Reunion is a festival on the island Éilís Mór. Miss Foster, the schoolteacher, explained the festival to Wakefield:

"It takes place when the constellation Reunion is visible in the sky. On those nights, dark things and spirits ride the winds that travel across the hills. As you may have already seen, at sunset the villagers don masks. They do this so that the spirits will not know them or carry them away to the depths of their underground caves."
"As I said, people dress up to protect themselves from those that will come. But this is not enough. Since the mirrors they have looked into while changing know their identities, they bury them next to the old fearnóg. The roots of this tree grow in both worlds at the same time, so they keep the witness mirrors safe."
Miss Foster

The children of Éilís Mór do not necessarily enjoy the festival. When Wakefield met some boys playing in the hills, one child mentioned the fearnóg tree in the graveyard:

"That tree gives me the chills these nights. On Reunion they got them little lanterns on it and they look like evil eyes staring into you."
Boy in the hills

The Lighthouse keeper was openly contemptuous of the festival:

"What do you think would happen if you unburied them mirrors? That a band of goblins would come out of the marshes to steal them? Nothing but nonsense."
"One thing I'm sure of, this 'Festival of Reunion' is nothing but a farce."
Lighthouse keeper