Completing my citizenship application

Hi,
So happy to join this forum. I'm in the U.S. and am applying for Irish citizenship because my grandparents were born there. I've paid for the application online, and but the instructions are that you must mail in your documentation. When I took my package to FedEx, they strongly advised against my sending it since I only have a P.O. box. The Office of Foreign Births for the Republic of Ireland only provides a post office box.  I used their web chat system to ask, but they would not give an address. They gave me a phone number to call and warned me that the wait times are very long. When I called, the recording said to hang up and try some other time. From what I can see online, the Office of Foreign Births is within the Republic of Ireland's Dept. of Foreign Affairs and Trade. I found a street address (42-47 Lower Mount Street, Dublin 2) for that department, but Foreign Births could well be located in another building. Do you have any advice as to how I can track down this street address? It took a lot of work to assemble the documentation; I don't want the package to go astray. Many thanks!
Pat