This from Gene Clifford Haley's "Topography of the Táin Bó Cúailnge", (Thesis), Harvard, May 1970:
This 'land of Níth' seems equivalent to the modern Co. Louth, and the story itself seems another version of the Battle of Crinna.
In LL 19 b 2 (Do Fhlathiusaib Hérend) we read of the eruption of the river, here called Níth nemide nemannach and glossed i nMaig Murthemne. According to the RIA Dictionary, némannach means 'abounding in pearls, or white pebbles', and so the phrase translates 'heavenly, pebbly Níth'.
Because of the huge associations between heaven and earth already explored both here on Mythical Ireland and in Island of the Setting Sun, it comes as no surprise to hear this "land of Níth" means "heavenly, abounding in pebbles Níth". Here we visualise the myriad of stars of the bright bands of the Milky Way (Bealach na Bó Finne) represented as pearly pebbles.
Níth is the name of a river also, the eruption of which echoes the eruption of the Boyne from Nechtain's well. Because the Boyne is obviously named after the heavenly river (Bó Finne), it could be reasonably speculated that the Níth story is a variant. Again, abundant in pearls sounds like a poetic description for all those pinpoints of light.
(My thanks to Alan Hand, Louth Co. Library, for assistance)