yes - probably very farfetched - you will I'm afarid be very disappointed when you see St John's - it was then little more than a hamlet (maybe 8 houses ?) surrounding the chapel but you can get a pint at Tynwald Hotel opposite Tynwald Hill
The Royal chapel wasn't 1557 - see http://www.ee.surrey.ac.uk/Contrib/manx/parishes/gn/german.htm

The more famous St John was misstress of Sir William Hillary but that was early 19th