Maughold parish reg. has:
John Garret & Ann Radcliffe married 20 Feb 1787. John from Lezayre.

Daniel Garrett & Catherine Cortean married 9 Nov 1816.

Definitely "Cortean" in the Maughold par. reg., but under Costean in the Ramsey par. reg. bapts., as you say.

There was a Cath Corteen bapt Maughold 8 July 1798 dau of Wm Corteen & Mary Kirmeen, and a Cath Costean bapt Maughold 19 Oct 1794 dau of John Costean & Ann Kirmeen.

An 1851 or 1861 census entry should answer the question of which Cath.

Sue