The 1911 Irish Census is online, absolutely free, and fully searchable for all 4.4 million Irish men, women and children of the time.
You can search by first or last name, of course. You can also focus searches on a given county, address or DED (election district). You can also specify gender and approximate age to help narrow down results...especially useful when searching on a name like, Oh, say....Fitzpatrick!
Saints be praised, it's Michael Fitzpatrick and family.
The search results make it very easy to quickly scan all members of a household, and even pull up an image of the original census forms. And let me emphasize forms...there are a number of different forms use to record details not only of people, but of structures and neighborhoods. Make sure to give them all a good looking-over.
There is also a very lovely section titled What was Ireland like in 1911? It's very much worth spending some time here (even if you don't have a single drop of Irish blood in your family line).
And whatever you do, don't overlook the fantastic picture of The Titanic under construction in Belfast.
See an earlier post for more UK Census and vital records.
Visit the main page of Free Genealogy Tools for more, umm, free genealogical tools.
And don't forget to also check for your family history at Ancestry.com and NewspaperArchive.com. These are subscription databases, but they are among the most powerful research tools available for looking into family roots.