hi, fluent Gaelic speaker here, I can confirm that "An Cuan Siar" means "The Western Ocean" and is the usual name for the Atlantic in Gaelic. Cuan in modern Gaelic means ocean (or a larger sea), but yes it was used for harbour archaically.
The Atlantic has other names such as "An Tabh" and "Na h-Abh", used in the Outer Hebrides, but they're not as widely used. "Atlantaig" is not used.

I've only heard Cuan nan Innseachan for the Indian Ocean, but both may well be in use