John SHEEHAN -son of Patrick, county and parish unknown

My great grandfather, John SHEEHAN was born in Ireland to Patrick SHEEHAN and Mary ?   circa 1851-1855.   His death certificate says his mother was Mary HALEY and he named one son Jame HALEY SHEEHAN, but as this is a secondary source, I use it as a hint, not gospel.  

Family lore says Patrick SHEEHAN may have died when John was a baby c. 1851-1856?  and Mary SHEEHAN when he was 12, circa 1863-1868, but one family member thinks it could have been reversed with his mother dying first.  One person also thought she had heard that Patrick died delivering mail or being kicked by a horse. 

He emigrated 1867-1870 and is first definitively found in Lawrence, Massachusetts at the time of his first marriage in 1875 when he says he was 24 years old. 

There were no known brothers or sisters to John. In the US, John 1st married a Margaret Cosgrove (Cosgriff) whose family can be found on various websites and were from Cloyne, Cork.  (Could he have known them before he emigrated?

A witness at several life events was a David Horrigan (Horgan, etc).   Could he have been a friend from Ireland?

I have run myself ragged for over 40 years trying to find out more about John SHEEHAN's origins.  I have all pertinent info from vitial records, obituaries, and so on, but nothing gives a hint as to whereabout of his birth. 

I would appreciate any insight on this problem.