Your message was posted a year ago, so you may have already tracked down the information you need, but I have a James O'Callaghan in my tree on Ancestry.com, who was born around 1820 in Ireland, married Margaret McBride, and died in California in 1866. If my James is the same as your James, my great great great grandfather, Owen, was his brother. I have the names of his parents and siblings, and their descendants if you would like access to my tree.