This is the dialect of English as (or was) spoken on the Island - basically a product of the late 18th and early 19th centuries when English began to replace Manx as the main language of the population.
Blundell, who stayed on the Island during the 1640's distinguishes the gentry from the people:
For their gentry are truly gentle, courteous, affable, and more willingly will discourse with you in the English than in their own language, whom I observed even of all them, not only to speak true English, but to pronounce so naturally as yt I cou'd not observe any different tone in their pronunciation of our English as is comonly noted, both in the Irish, Scots, and Welsh, and in all strangers, neither any of these to be distinguished from our English, either by the countenance, carriage, apparel, diet, or housekeeping, but in most imitating, as Speed well observed, ye Lancashire gentry, as having had so long converse with the house of Darby themselves and all their officers and retinue being all Lancashire men.
Thus to Blundell, himself a Lancashire man, the Manx dalect was undetectable.
The first reference I have seen to a recognisable Manx dialect is that of Lord Teignmouth, writing of 1829:
The English spoken by the lower classes of the Manks is, on the whole, pure, and the pronunciation is free from provincial corruptions. As it has not acquired the settled form and consistency of an ancient dialect, but has been recently borrowed from the surrounding countries, its phrases, and the tones of the speaker indicate a singular compound of the dialects of the north of England, of Scotland and Ireland. The words " indeed," "at all," " convenient," are used after the Irish fashion.
T.E. Brown says in 1895 writing to Rydings :
I have an idea that Mr. M.'s new book will show plainly that we have arrived at the last squeak of the Manx language proper. So I think what we have now to do is to make a new start, making Anglo-Manx dialect the basis. In its turn this will probably become obsolete, but meanwhile the catastrophe will be deferred by your stories, and, perhaps I may add, mine.