:) the name for Atlantic ocean sounds similar to the name for Ireland, I'm guessing for obvious reasons?
I would guess (only a learner myself) that Iwerddon might have been Iweryddon sometime in the past - perhaps something like the land in the ocean, or that the water would be the water around the land or something.
I always say 'Cefnfor' for Ocean and 'Mor' for Sea, yet, entering Sea was denied. Any particular reason?
Just tradition, I think. The usual English name for it is 'the Atlantic' or 'the Atlantic Ocean'. In Welsh the word Môr is usually used as part of a name of a sea/ocean rather than Cefnfor.
One of the difficulties of translation.
In Welsh a proper name doesn't need a definite article.
The literal translation of 'Mae Môr Iwerydd yn oer' would be 'Atlantic Ocean is cold' , which needs to be transcibed to 'The Atlantic Ocean is cold' to be correct.