The 1901 and 1911 Irish census records online

It used to be that you had to know what townland your ancestors came from, in order to be able to find them on the Irish census records that were kept in Dublin and Belfast. Then, to the great joy of all those with Irish ancestors, the 1901 and 1911 Irish census records were made available online.
You can search the 1901 and 1911 census records for free at http://www.census.nationalarchives.ie/.
Unfortunately, earlier census records no longer exist, but there are a number of substitutes, of which the Griffiths Valuation is the best known. You can search it free at http://www.askaboutireland.ie/griffith-valuation/
Published between 1847 and 1864, Griffiths was the first full scale valuation of property done in Ireland (to determine liability to pay the Poor rate for the support of the poor and destitute within each Poor Law Union) and provides detailed information on where people lived in mid-nineteenth century Ireland and the property they possessed.
Griffiths can help you identify a parish and townland in which your ancestors may have lived, thus helping you identify church records to consult for further information.

 

I recently discovered that PRONI has made available images of the Valuation Revision Books that have been used to track changes in land ownership since the original Valuation was done.
You can look at books for Counties Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone between 1864 and 1933. In all, over 440,000 images are available to view online. (You also can save a particular page, which is extremely useful.)
You can search by Placename (City, County, Parish, Townland) or PRONI reference. The cities of Londonderry and Belfast have been indexed to street and ward level; streets can be found by using the free text search.
This facility is extremely useful if you know the townland or street in which your ancestor lived; you can then follow the changes in ownership of their land throughout the years.
Valuation Revision Book Search