Make it plausible: The author writes a letter about Irish English, and he says he wrote a letter to make it more real and therefore more interesting to read.
Be knowledgeable: The uses of names such as "James Joyce" and "J.M. Synge" shows that the author knows about the topic, also how he refers to articles of the Constitution represents his knowledge of the theme.
Keep it Relevant to Anglophone Culture: The letter and rationale are clearly referring to topic relative with the United Kingdom.
Know the right conventions for the text type: the author knows how to write a letter properly.