Dumnonii

A tribe of Celtic Britons on Earth. The Dumnonii controlled the southwestern peninsula of the island of Britain. They lived in lowlands and in valleys, in which manner they named the land Dan-munith.  
Dumnonii is thought to mean 'deep valley dwellers', from the Proto-Celtic dubnos, 'deep'.
The territory of the Dumnonii later became the Brittonic kingdom of Dumnonia.