Do you know the names of the siblings to William Henry O'Brien who were also born in Ireland?
I searched on the subscription site Roots Ireland and do not see a baptismal record for a William O'Brien who was born and baptized around March 25 1851 and had a father William. I see a March 9 1851 baptismal record in St/ Peters and Paul in Waterford town father William O'Brien and mother Mary Darigan. There are no 1851 baptismal records where the mother is Ellen or Eleanor. There are records in 1848 1849 and 1852. The maiden names were Leary, Menton and Donnell respectively.