DOMNU.  Ancient Celtic name, possibly meaning "goddess of the deep."  But it may be masculine.  See note below.  And see Dumno and Devon.

... The conjectural interpretation offered to you of the term Fir Bolg ("a bag-man") may be said to derive some confirmation from the name of their allies the Fir Domnann.  Now Domnann should be the genitive of a name making in the nominative Domnu or Domna; and construing Dé Domnann in the same way was Dé Danann, I take Domnu to have been the name of a goddess and not of a god.  Put it back into what must have been its early form, and you will have a nominative Dumnu and a genitive Dumnonos, implying a stem Dumnon: form from the latter an adjective Dumnonios, you will then have as its plural Dumnonii, the attested name of two peoples of Roman Britain, situated respectively by the Severn Sea and the Firth of Forth.  The name meant a people who had something to do with Domnu, and that something may have been a claim to be the descendants or the favourite people of Domnu; while Domnu's own name, derived from the same origin as the Celtic words for "deep," probably meant the goddess of the deep.  Thus two historical peoples and one mythic had their names from this goddess, of whom nothing is otherwise known except that she was the mother of Indech, one of the leaders of the Fomori, and that several waters were called after her, such as Inver Domnann or Broadhaven, in the county of Sligo, and Malahide Bay, near Dublin (p. 583).  In this last respect she resembled the Dee or Aerven, as she may have done in others likewise. (The Hibbert Lectures, Trust, 1888)