The Encyclopedia of Pennsylvania, Biography, by John W. Jordan, LL.D., Vol. III, New York, Lewis Historical Publishing Company, 1914, page 872, states:
“Laird” McClellan, founder of the Chester county (Pennsylvania) branch of the family, was of Bannagachen, Ireland, and in 1685 was banished to the American colonies on account of the part he had taken in the wars. He was accompanied by three of his children and the family settled in the New Jersey Neighborhood, where they remained until 1689, when news of a favorable change in affairs at home caused the “Laird” to resolve to return. On the voyage he was taken prisoner by the French, but finally arrived at home on the last day of October, 1691. The children remained in America and became the progenitors of the Chester county branch of the family.
The story related would seem to be identical to that of Robert McLellan of Barmagachan, Parish of Borgue, Kirkcudbrightshire. See the article on the Kirkyards Website.
There is no question that the McClellan family were established in Chester county at an early time (not long after 1700), see for instance History of Chester County, but there seems no evidence to link them to Barmagachan. Does anyone have any?