It offered me "Mae'r Môr Iwerydd" and "Mae Môr Iwerydd", and dinged me for not checking both. Is the definite article optional here?
Had the same question myself, and then saw ibisc's comment here: "With proper names such as 'The Atlantic Ocean', 'The National Assembly of Wales', and so on, y is not usually included in the Welsh."
No "The" ?
Yes. It's the 'r after Mae.