The important part for answering this question is: "Believe Me, if All Those Endearing Young Charms" is a popular song written in 1808 by Irish poet Thomas Moore using a traditional Irish air.
This justifies what answer for what question? Q & A:
Passage: "Believe Me, if All Those Endearing Young Charms" is a popular song written in 1808 by Irish poet Thomas Moore using a traditional Irish air. Moore's young wife had been stricken with an illness and worried that she would lose her looks. He wrote the words to reassure her.

Question: Based on this passage, who wrote believe me if all those endearing young charms?
Irish poet Thomas Moore