Hi ..this information may not be what you want, but any way. Maybe even they are different William Costello's as someone said there are many in Cheviot. They sound similar apart from date of birth and wife. They are from the same place in Ireland though.
I have been researching about the Costello's. William Costello, born 1840 is my great -great Grandfather. He emigrated from Ireland to Cheviot, North Canterbury, NZ. He married Eliza Jane Dalzell, born 1858, Northern Ireland, who moved to NZ. Him getting married to someone else is likely, but Eliza and him had 9 children together. His son Thomas had a son Thomas (my grandad), who had a son Thomas (my uncle) for a change. The Costello's are still farming around North Canterbury, NZ in Hawarden (close to Cheviot). My grandfather will know a lot more about William. I have a very large family tree though.
Eliza Jane Dalzell was my great great grandmother and I have a wonderful picture of her. If so it may be a picture of William Costello's mother.