Just a quick correction, for Fullon. The acronym IRS actually stands for Internal Revenue Service, and not Inland Revenue Service. The definition can be found at wordnetweb.princeton.edu from that, the other information in your post is spot on. Might just want to research the true name of an acronym-ed Organization before you post it.
The IRS or the Inland Revenue Service is the United States Government agency for the collection of taxes and enforces the laws related to internal revenue. The headquarters of the IRS is located at 10th St & Pennsylvania Ave, NW, Washington, DC 20004. The building of the IRS is located at the intersection of the Constitution and Pennsylvania Avenue. The building of the IRS overlooks the museum and monuments of the National Mall. Unfortunately the building is not as such open to the general public and entry is by appointment only. The mailing or postal addresses of the Inland Revenue Service are however different from its headquarters and are based on several categories of returns being filed. These addresses can be found from the website of the IRS.