In the census records, he is fairly consistent about his age, depending on the date of the census -- he seems to have been born 1837-1839. I suspect the passenger record you found is likely him -- he may have made himself of age (ie, 21) so he wasn't returned to Ireland.I did find an entry in the Iowa marriage index:
name: Hugh Mckinstry titles & terms: event: Marriage event date: 27 Mar 1870 event place: Humboldt County, Iowa, United States age: estimated birth year: father: father's titles & terms: mother: mother's titles & terms: spouse: Frances J. Blackman
The original record may provide more clues.Most of the census entries also indicate that his parents were born in Scotland, so while he may have been born in Ireland, your roots lie in Scotland.
He lived in Iowa as late as the 1900 census and then moved to Colorado.You should follow up to get his death certificate between 1920 and 1930 and hope that it names his parents.