I searched on the subscription site Roots Ireland and only found one baptismal record for a sibling to Walter. Mary Saunders baptized May 6 1838 at St. Canice's
However, when I searched on Sanders, I found two more records:Patrick March 11 1836 and Bridget June 7 1846. (As an aside, I would have thought the Roots Ireland search engine would have searched Saunders and Sanders at the same time).
I did not find the marriage record.
The Saunders family lived in Deerpark townland in St. Canice's civil parish.
FYI. The Bridget Saunders/John Long marriage was at St. Canice's February 16 1863
There is a baptismal record for the elder Walter and the info is as you describe in your message.
I looked at the 1901 census for Deerpark. There is one Long family. Father was John but Mother was Johanna not Bridget http://www.census.nationalarchives.ie/pages/1901/Kilkenny/Kilkenny_Rural...
1911 census again Johanna http://www.census.nationalarchives.ie/pages/1911/Kilkenny/Kilkenny_Rural...
Let me know if you have any questions.