IGI has Rob. Moore marrying Jane Cowley at Braddan, 13/11/1858. Also has 4 children
Margaret Jane c. 29 dec 1861, Cathrn Ann c. 26 jun 1863, Elizabeth Eleanor c. 12 feb 1865, Robert Stephen c. 7 Oct 1866.
Suggest you ask for a 1861 census look-up to find this couple to ascertain approx. age and place of birth. Then using the IGI find the best fit baptism records and cross reference parents and William. Also any daughters names marrying a Cavendish.
If he was in US 1870/1880 census that would also help confirm his age.