We've waited patiently (or not so patiently, if truth be told) for these releases and now we can start ticking off the days.
The National Archives of Ireland's collections of 19th-century census fragments and census search forms will be uploaded to the NAI's free Genealogy website on Monday 28 April. They will be added to the databases of FindMyPast and FamilySearch at the same time.
The 19th-century census fragments are the surviving parts of the diennial censuses taken 1821 to 1851. The release of these records will, I'm sure, come with background information, but in the meantime, you can take a look at the overview on my website, Irish Genealogy Toolkit. I don't know if the release will include the surviving transcriptions of the 1861 Enniscorthy census and/or the 1871 Drumcondra & Loughbracken census (more about these on Toolkit here).
The Census Search Forms were the documents completed by those wishing to 'prove' their age to support an application for a state pension, following its introduction in 1908. A search was made of the paper returns for the 1841 and 1851 censuses (which had not been destroyed at this time) to see if the applicant could be identified. Whether 'found' or not, these search forms often hold terrific genealogical information, as information they provided about their parents and siblings was often noted on the form. They're often referred to as Irish pension records, but as the collection becomes more readily available, they will probably become better known by their correct name of Census Search Forms in the future.
Again, I'm sure the NAI's Genealogy site will include explanatory text about the Census Search Forms, but while we're counting down, you might like to read an overview on my website here.
Not long to wait, now. Tick-tock.
UPDATE Monday 28 April: Sure enough, the pre-1901 census fragments & substitutes launched.