Colin, MI means Monumental inscription (on a grave stone). They have been transcribed by volunteers from the IoM Family Hist Soc., and published by them.

Jean looked up the IGI on FamilySearch.org. You may have missed the entries if you put in the full name for Mary Creer under "Mother", if only her forename was given in the parish register. She looked for "extracted" entries (from the parish register), rather than church members' unreliable submissions.

It looks as though this IGI extracted entry is a possibility to be for your Peter Corkill:
PETER CORKIL Christening: 09 AUG 1789 Onchan, Isle Of Man, Father: FINLO CORKIL.

Sue