Complex sentence: Dicuilus claimed to have met some Irish monks who had lived on the island of Thule, where they said that darkness reigned during winter but summers bright enough to pick lice from one's clothing.
Split into simpler sentences: Dicuilus claimed to have met some Irish monks who had lived on the island of Thule. They said that darkness reigned during winter but that the summers were bright enough to pick lice from one's clothing.