I must admit Chucky that I also thought of the possibility that William Snr wasn't born on the IoM but the 1861/71 and 81 census years also list his birth place as Rushen, Isle of Man
Going back to my original suggestion re a baptism in the parish with father Martin Norway and mother Cath Kiggen - I did find a marriage in 1838 at Rushen of a Cath Keggin to John Collister
https://www.familysearch.org/ark:/61903/1:1:XZ1C-J82This looks like the couple in 1841 living at Corvalla, Rushen, Isle of Man with her parents and son William
Edward Keggeen Male 60 Isle of Man
Jane Keggeen 63 Isle of Man
John Collister 35 Isle of Man
Catherine Collister 35 Isle of Man
Wm Keggeen 14 Isle of Man
In 1861 William Northey and family were also living in Corvalla, Rushen next door but one from Jane Keggeen and her daughter Catherine Collister. Maybe the father John on William Snr's marriage cert was incorrect and William's stepfather was John??

Kay