I don't know how much interest you have in Michael. You could try to get a copy of his death certificate which should have his parents' names. I found Michael Kerwin who died June 20, 1912 in Manhattan, age 75, in this database: http://www.italiangen.org/NYCDeathSearch.asphttp://www.italiangen.org/NYCDeathSearch.asp
Michael received a Civil War pension Appl 219,928 Cert 163,733 Widow Catherine Appl 997,749. The widow's application was in 1913 which would indicate he died before then. She didn't receive a pension certification which probably means she died before the certification was completed. Copies of the CW pension files can be gotten from the National Archives in Washington, DC for $75. While a bit pricey and the genealogical data it may contain varies greatly it may have everything you ever wanted to know about him. Since he was born in a foreign country it would most likely have info on his place of birth and arrival in the U.S. As I understand it the CW pensions will all eventually be online but I don't know the timeline. However some are already online for at Footnote.com for a fee.
He probably got married in New York and the record of the marriage may have info on his parents. The pension file should have when and where they got married. Catherine had to prove she was actually married to Michael. He was a naturalized citizen, his papers could prove helpful especially in determining when and where he arrived. They are probably on file in NYC or Philadelphia.
1850 census, Philadelphia
John Kevarre 37 Catherine 34 Michael 15 Margaret 12 Mary 10 Elizabeth 1, born in PA all the others in Ireland
1860 census, Philadelphia
John Curwin 47, born Ireland Cath 44, ditto Margaret 21, ditto Elizabeth 10, born in PA Mathew 8, ditto Jno 3, ditto
1870 census Philadelphia
John Kerwin 60, born Ireland Catherine 54, ditto Elizabeth 18, born PA Matthew 18, ditto John 12, ditto
1880 census, Philadelphia
John Kerwin 70, born Ireland Catherine 66, ditto Elizabeth 26, born PA