The days of pulling your hair out when you want to find a historical directory for a particular place are over!
A collaboration between Joe Buggy, an Irish genealogist (and blogger) now based in Washington, and Shane Wilson, a family historian based in Dublin, has seen the creation of a database of more than 550 links to online directories. These directories are scattered across the web, held on both free and subscription sites. Some are free, some are not. Some are available for purchase on CD, others for download.
The Irish Directories' Database is held on Shane's website (a great resource for those with Dublin ancestors) and is provided free for the benefit of all researchers. Definitely one to bookmark.