This is where you will find links to web sites which contain information about passports as they pertain to genealogy. This includes information about how to use passport information, the information itself, and articles about the correlation between passports and family trees.
Regular Websites
About.com: Irish Citizenship by Birth
Tutorial written in simple terms walks the reader through researching Irish ancestry, specifically how to use passport information to do so. - About.com: Irish Citizenship by Birth
http://genealogy.about.com/od/ireland/a/citizenship.htm
The National Archives of the United States government puts out this article which presents information and search strategies for tracking down the passports of relatives in times past. - Passport Applications
http://www.archives.gov/genealogy/passport/
Search Passport Applications 1795-1905
Footnote offers a database of letters and applications presented for issuance of United States passports from 1795 to 1905. - Search Passport Applications 1795-1905
http://www.footnote.com/documents/52163237/passport_applications_17951905/?xid=275