Probably named after Watermouth Castle, a country house in North Devon, England. This is supported by the presence of Clovelly Street nearby, named after another place in North Devon. The connection may be a literary one, as both places feature in the writings of Charles Kingsley (1819-75), author of The Water-Babies.
"There shines among the woods the Castle of Watermouth, on its lovely little salt-water loch, the safest harbour on the coast; and there is Combe-Martin..." (Kingsley, Prose Idylls, New and Old, 1873)
The Irish language form only applies to part of the street.
Unfortunately, the official Irish version of the name fails to recognise Watermouth as a place-name and translates the elements piecemeal, when the name would be better left untranslated.