I recently learned the word cefn , for 'back, rear'.
Since -for sounds as a softly mutated môr , does the word for 'ocean' literally mean 'the sea at the back', 'the sea behind' (likely, behind Ireland)?
I was wondering this, too. I think it might be similar to using the term backcountry for remote countryside, as the Welsh for countryside is cefn gwlad.
I think you're on to something!