Ancestry gives the following death record for a miner named Cornelius Sullivan living where your Cornelius lived in 1870. This Cornelius is a bit older than the age given in 1870.
Name: Cornelius Sullivan
Birth Date: abt 1827
Birth Place: Ireland
Death Date: 13 Jul 1879
Death Place: Central Mine, Keweenaw Co., Michigan
Death Age: 52
Occupation: Miner
Race: White
Marital Status: Married
Gender: Male
FHL Film Number: 1310167
In the 1880 census, Mary Sullivan is a widow, with five sons. She has moved about ten miles south of Central Mine.
Name: Mary Sullivan
Age: 40
Birth Date: Abt 1840
Birthplace: Ireland
Home in 1880: Calumet, Houghton, Michigan, USA
Street: New Town Elm Street
Dwelling Number: 233
Race: White
Gender: Female
Relation to Head of House: Self (Head)
Marital Status: Widowed
Father's Birthplace: Ireland
Mother's Birthplace: Ireland
Occupation: Wash Woman
Months Not Employed: 12
Neighbors: View others on page
Household Members Age Relationship
Mary Sullivan 40 Self (Head)
Jamess Sullivan
14 Son
William Sullivan
12 Son
Cornelius Sullivan
10 Son
John Sullivan
8 Son
Timothy Sullivan
6 Son
According to Irish naming patterns, the first son is named after the paternal grandfather, the second after the maternal grandfather, the third after the father. Note that none of the children here is called Jeremiah. Therefore, I believe the baptismal record you located is probably not for your ancestor.
About Julia Moynahan--on Ancestry, I saw a marriage record for one of Cornelius's sons; he was married to Julia Moynahan.
Mary Sullivan's maiden name was Fitzpatrick, according to one of her son's marriage records on Ancestry.
I hope this is helpful..