I haven't mentioned the National Historical GIS data base before here, but I used it recently and found it very easy to navigate and I know that there are loads of people out there who want to use old census data for mapping purposes.
First off, here's what the site says about itself: "The National Historical Geographic Information System (NHGIS) provides, free of charge, aggregate census data and GIS-compatible boundary files for the United States between 1790 and 2000."
By providing your email address, you can get boundary files and data for all years that U.S. Census has been performed.
Here's a cool thing I found out, if you made a search for a particular set of data for a certain geography, you can go back and review your Extract History (because you've given your email address) shown in the red circle below. Even if you made your search a year ago, it'll be in the Extract History:
And then you can change either the Geography or the Data tables and make a new search/data extraction. This saves a lot of time plus you do not need to remember which data tables you got when you want to compare two or more geographies for the same time period.
The data available are quite interesting. How people were categorized in the past is pretty different than today, as we know. I believe this site has been up awhile but it is worth mentioning it. The data go down to the tract level, so make note on that. You won't really be able to locate an Irish neighborhood but this is still a very useful resource.