I'm still back-checking myself to clarify why I thought William was born in Castletown. I see that you're right on the 1841 census -- that Castletown is where he was at that time and birthplace is not noted. I see where Onchan/ Begoade is located about 5km north of Douglas. I wonder what would have brought Jane Cottier (from Peel) and William together. I suppose it's as likely from Onchan as from Castletown. I have noted several marriages between Christian and Cottier family members on FamilySearch.com. William and Jane's marriage registry entry (27Nov1825) lists him as from Malew parish and she from parish of German). But that doesn't verify their birthplaces.