The FAA allow aircraft registered in the USA to be flown by either FAA Certificate holders, or the holder of Licence issued by the country over which you are flying.
The IOM is not officially part of the UK, or the EEC, and although it will validate other Licences to fly M registered aircraft, it does not, as far as I know, issue it's own licences, relying on the UK to do that on it's behalf.
As the IOM is not a separate country, for the purpose of Licence issue, I would say that you would have to have either an FAA Certificate, a UK National Licence, or a UK issued EASA licence to fly an N registered aircraft over the IOM.
MJ
