I think we have to be very careful when we interpret documents.
My own feeling at the moment is that there were three births around the same time, two of them (Jurby) had the same father but different mothers.
Was Mary Ann Curphy the same as Ann Curphy.
While the father's name may have been omitted by mistake, this would be a very rare case. Can anyone come up with a similar case in any of the Parish Registers? I think it so unlikely that I would need incontrovertible evidence in order to accept it.
In similar manner, I think the likelihood of someone being enumerated on the Island and on the Mainland is close to nil.
An intresting puzzle. Good hunting.