George Hunter (1745-1830)

Does anyone have any information regarding the parents of my ancestor, George Hunter?  We have records of his living in Pennsylvania, and Ohio, and his marriage to Sarah Paxton in 1785.  We believe he came to this country as a young boy with his two brothers, James and Joseph around 1751, but have no records of his arrival.  The records we do have says he came from County Tyrone, Ireland.