Hi Averil,
The only c1765 Maughold bapt I can see who could possibly be Catharine later called Cowle is:
Catharine Illegit daur of Jane Cashin bapt 12 Oct 1765.
Father not named, but illegitimate children in the IOM were often later known by their fathers' surnames.

How do you know she was bapt in Maughold?

Sue