Where did you get the information that Thomas CONN's father was William and that William brought the family to Washington Co. PA? Thomas' oldest son was James, who by Scots-Irish naming tradition should have been named after Thomas' father.
According to the 1860 and 1870 censuses, James was born between 1795 and 1797 (age 65 in 1860 and 73 in 1870). In both censuses, James says he was born in Delaware, which is probably just the place of his earliest memories. I believe that his son George W. is correct when he says that some of Thomas' older children (James and Jane, at least) were born in Ireland.
James' sister Jane (COWAN) says in the 1850 census that she is 53, born in Ireland (1797). Unless she and James were twins, he was probably older than she.