usually the parish in which a child is christened is that in which his parents are residing at the time of birth - however no guarantee of this.
Also remember that the place of birth may be wrongly remembered - eg farm labourers might well move nearly every year and the children may well grow up in a different parish to their place of birth - also if they are servants their master might well indicate the current parish of residence of their family.

Re Castletown - this is wholy in Malew