Interesting discussion - I suggest what you are actually seeing is the Anglicisation of the Island - the Manx language was dominant in the rural parishes but declined in the towns (tho Castletown was always an English town) from the 1720s as more 'strangers' appeared with the rise of the offshore trade - the 1780 saw the rapid rise of Methodism esp in the North which tho in many ways was a response against this Anglicisation actually encouraged it as it saw the use of English ministers even tho many of the lay preachers were Manx speakers - in many ways the emigration Ohio saw these strong Manx speakers emigrate.