How do you address the Prime Minister of Ireland?

Looking at these sites plus using what I know about current letter usage, I learned that you write:

Dear Prime Minister,

I am concerned about...

Sincerely,

Jane Smith

You can replace the Word Sincerely with some other correct Word. However, that simple Word is correct and current. The punctuation has also simplified to the commas as in this example. You do not use Mr., any other honorific, or the prime minister's last name.