John Brett or Britt

I am trying to verify the origins of John Brett, or John Britt.  Family stories say that John was born in Kilkenny on 10 Sept 1839.  I can't verify this date, but the ages given on his marriage and death certificates indicate that he was born around 1840. Whether 'Kilkenny' means the county or the city is not apparent. 
 

John apparently arrived in South Australia around 1864, but once again this cannot be verified.  He really is a mystery man!  He was definitely in SA in January 1871, when he married Ellen O'Neill, who came from Tipperary.  He was Catholic and they had eight children together before Ellen died. He remarried in 1891 and had four more children before dying at age 57 in 1897.  He gave his father's name as Percival Brett.

The family has largely used the name Brett in Australia, but some of John's children were baptised Brett and others Britt. We discovered that John had a brother who went to New York in the US, whose name was Pierce Britt. Looking on Roots Ireland and Ancestry shows no birth record for John and certainly no records of any sort for Percival Brett.  I suspect that the family name was Britt in Ireland and that Percival has been corrupted from Pierce. There are baptism records for Pierce Britt in Kilkenny, one at St Canices (1831) and another at Ballycallan-Kilmanagh (1838).  The St Canice's record is promising because the father is also Pierce Britt and the mother Catherine Croake.  John's eldest daughter was Catherine. The address was Black Abbey.  
 

The other  record has parents Patrick Britt and Mary Corcoran, which are not family names.  Family names include Catherine, Margaret and Ellen for girls; Percival, William and John for boys.  John also had another brother called William and Margaret was also Ellen's mother.  The issue is how to verify any records we find and what else can we use to substantiate we have the right people. There do seem to have been quite a few Britts in the Ballycallan-Kilmanagh area around that time.

I know this is not a lot to go on, but can any one suggest we're I might go to find out more information about John and Pierce? 
Sally