I went to www.irishgenealogy.ie which is a free Irish government site which has records for the Diocese of Cork and Ross.
I found an 1853 marriage in the Cork South parish between a Michael Sullivan and a Catherine Casey. Perhaps this is the other couple that you referenced.
I also found a February 17, 1874 marriage in Bantry for Michael Sullivan and Catherine Casey. Michael was shown as coming from Drimoleague and Catherine from Caheranacerin. ( I will look up where this townland was located).
I also found three baptismal records in the Drimoleague RC church for children born to Michael Sullivan and Catherine or Kate Casey:
Anne May 27, 1875
Daniel February 23, 1877 NOTE father and son shown as O'Sullivan
Hanora April 8, 1879
Perhaps the mother died giving birth to Hanora or sometime after. Michael and Daniel emigrated and maybe the sisters stayed with family in Cork.