Actually it was Donna who provided the info on the German marriage. The IGI entry was submitted by a member of the LDS church and also gives an approximate date of birth of 1778 for William and 1782 for Mary.
THE IGI records of the parish of German show three William Corkils christened between 1775 and 1885
March 21 1779 son of William Corkil and Jane Callow
Feb 9 1782 son of William Corkil and Elizabeth Kissage
Aug 21 1785 son of John Corkill and Catherine Cabbon
There is one Mary Kneen
Oct 10 1779 daughter of Edmund Kneen and Cath Cottiman

The most accurate method of identifying parents is by wills. You may want to check Brian Lawson's site and order the film at your local family history center. I noticed that an Edmond Kneen has a will in German in 1782.

However the in the late 1700s people moved very little, unless they could afford a horse.