So I managed to locate several federal census records for my great great grandfather. The 1920 census shows John Slowey and wife Theresa, ages 58 and 54 respectively, still living with two sons, Clarence and Clement (ages 17 and 15 respectively) in Mayfield Township, Yankton Co., South Dakota as of January 10, 11, and 12, 1920. It shows that John is a grain farmer and son Clarence is a farm laborer (likely working for dad). John was born in Wisconsin and Theresa was born in Missouri (aha! more info on Theresa!). Both boys were born in South Dakota. We also know from this census that John's and Theresa's parents were born in Ireland. Another clue!
The 1910 (May 4, 1910) federal census for Mayfield Township, Yankton Co., South Dakota shows our beloved John and wife Theresa, but with many more children!
Peter, Mary E., Theresa, Ellen, Thomas P., John E., Clarence, and Clement - ranging in age from 5 to 21. Obviously, six of the children flew the coop over the next 10 years. We'll have to find them on their own census records. We can tell from this census that both parents and all the children can read and write, and that John owns his home (with a mortgage). We can also see that John is not a survivor of the Union or Confederate Army or Navy (obviously because he was only a toddler during the Civil War).
You'll notice that the actual census forms are fairly difficult to read, so it's hard to determine what each of those columns identify. Ancestry.com has some handy-dandy census extraction forms that help determine what each column is for, and you can use them to transcribe the information you find (very helpful when you don't have the option to print or save to your computer/laptop right away). They can be found (for free) at: http://www.ancestry.com/charts/census.aspx.