Here's the blurb on the two useful databases available:
Irish Population Change Atlas
The Irish famine population atlas provides population data for all 32 counties within the Island of Ireland from 1841 to 2002. Data is available at Electoral Division level, at a consistent set of 3432 EDs, which is based on the 1851 Electoral Division boundaries.The data used in this population atlas is from 16 censuses, taken in both the Republic and the North of Ireland between 1841 and 2002.
Irish Famine Data Atlas
The Irish famine is a key event in Irish history. This online Atlas of Irish Famine Data provides users with access to population and agricultural data for Ireland for this period. Using data from the Census of Population 1851 and the Agricultural Census of 1841 users can examine aspects of the Irish human and physical landscape at this time in Irish history.
Also chcek http://ncg.nuim.ie for more useful background info on the projects, and for a link to another useful project, on Donegal Placename Mapping.
(With thanks to @dougangene and @sheentatit on Twitter)