The phrase "Môr Iwerydd" means "the Atlantic Ocean". Even though the word "cefnfor" can be used, "môr" is favoured in set terms like the above :)

yr Iwerydd = 'the Atlantic' as a noun.

Môr is used in the names of the oceans for some reason. Just convention, really, I suppose.

Thank you for your answers!

No, Iwerydd does not mean "Atlantic" in the sense of "related to the legendary figure Atlas from Greek mythology".

But the body of water which is called "Atlantic Ocean" in English is called Môr Iwerydd in Welsh.

The word for "ocean" is cefnfor but their names generally use môr "sea". Compare "the Seven Seas" in English, which also uses "sea" for oceans.

Welsh names are simply not just translations of English names.