Sophia Morrison puts it well
' God bless me, bogh, we mus' give thee a name!' said Illiam. And he took off his hat, and stood up in the boat, and threw a handful of water towards the child, crying out: ' If thou are a boy, I chrizzen thee in the name of the Father, Son, and Holy Ghost, Juan! If thou are a girl I chrizzen thee in the name of the Father, Son, and Holy Ghost, Joanney!'

You can look for patterns (eg naming after grandfather but I suspect they are not a 100 % reliable guide - English naming customs pre 16th century were to name after god-parents but by time of Manx records I suspect any coherent pattern had broken down and each family went its own way, some names will run in families, as a women under Manx law was the equal of the man the woman's family name often appears as a second name - however being an ex-academic 'more research is obviously needed' !
(see www.manxnotebooj.com/fulltext/sm1911/p034.htm)