Edward Corkill is indeed both Parish Clerk & Schoolmaster but he is not 90 when he dies viz-
Maughold Monumental Inscriptions
244
Edward Corkill Parochial Clerk & Schoolmaster of this parish 44 years d 17/5/1820 aged 69
Margaret Corkill alias Christian relict of above d 15/12/1842 aged 74
That gives him a birth date of 1751 but have not yet worked out who his parents are