Thank you very much for your query. You have already found out interesting information about your ancestors. Did the census records mention the name of the ship or the port they arrived at or a specific date? Usually information was taken at the place of arrival rather than departure. Details about your ancestors could have been taken and may include a more specific location in Cork. You should also consult the birth and marriage records for them. Marriage records date from 1845 in Ireland, but your ancestors might have been married after that year. You should find knowing the details you have already gathered very helpful for searching the birth records. When you do find the correct birth record, you might pick up on more clues about William Cross and Johanna Murphy.
In the meantime, keep checking the IRO website. Someone in the community might know more information.