Dear Finlo
My Ballacorkish great grandparents, Thomas and Alice Kneen, were related to both the Cain family and the Costain family being discussed in this thread, and they were close enough to have exchanged photographs of themselves and their children during the 1890s and early years of the 20th century.
Alice Kneen nee Clegg 1847-1931 had a brother Edwin Clegg 1861-1925, a dentist in Douglas, who married your great grandfather's sister Maud Cain 1863-1943.
Thomas Kneen 1846-1936 had a sister Margaret Kneen 1843-1914 who married Richard Costain 1839-1902, and one of their sons, John Kneen Costain b1876, married Mary Christian Cain b1889 who was a sister of your grandfather.
My cousin had been struggling to make sense of all these messages, so I've just sketched her a tree which you might find helpful. I have an email address which I use for genealogy purposes:
faircork-tree (at) yahoo dot co dot uk
and if you contact me, I can send the tree as well as photos of Maud nee Cain and of her 2 children who were first cousins of your grandfather.
Jean C