In the 1851 census the only Manx-born person residing in Cornwall is Thomas Corkill. He is living at/in St. Mawes, St. Just, Cornwall HO 107/1909:
Thomas Corkill widower 56 Coastguard, b. IOM.
Sarah Corkill 20 Milliner b Manaccan
Agness 16 Housemaid b Gunwalloe
Samuel son 14, Ap. to carpenter b Gunwalloe.

In 1861, Thomas and Agnes are still in St. Just. It looks as though Sarah may have married John Barnicoat, Carpenter in St. Just and Samuel is a carpenter in Islington.

Thomas's wife was called Agnes. There are three christenings:
Sarah Corkill was christened in Manaccan on July 14 1830, Agnes as christened in Gunwalloe on June 13 1834 and
Samuel on May 13 1836.

One thought is that, given the few Manx in Cornwall, Thomas was related to John (an orphaned nephew or a son by an earlier marriage?).

Note that there is also a John Blake Corkill who married a Melicent Avery in Forabury Cornwall in 1796.
John Blake Corkill, son of William Corkill and Jane, was christened in Whitehaven on April 18 1773. There may be a Manx connection there as well.