If the hypothesis that he was illegitimate is correct then he would most likely be always known as Casement assuming his father acknowledged him (and his mother wanted it)!

It is the IGI that is incorrect in applying English standards/Law to the Manx situation - the parish register most likely read x illegitimate son of y & z - however the English custom began to dominate as the 19th century wore on and of course the 1877 civil registration virtually compelled it.