I too am researching John Harley/Mary Cleland.My connection is through their son James, b. 1782.
I have the following, plus a little more info on dates of their other children:
John Harley came to Washington County, VA in 1798 from PA.Married for 51 years to Mary Cleland Harley (married Christmas, 1774) From 5/13/1958 letter to C. H. McKennon from Charles Standard Harley: "...Our people were in Pennsylvania as early as 1764, we know,and presumably before that, in Chester County.Harleys are as common there as Smith in Arkansas.The family had been in North Ireland since the time of Cromwell, or even before.That would throw the time considerably before the time of Robert Harley, who arose to importance under William and Mary, and became a member of the Cabinet under Queen Anne, after the death of William.My information is the family came to America about 1690, which would be a hundred years before Robert'stime."
Please let me know what other information you have; I will share what I have.