there are complexities everywhere - the parochial system was already 700 years old and did not reflect the late 18th/early 19th C growth in towns (especially Douglas)- a major change took place in the mid 19th C.

Small details matter - assumptions are dangerous - it would appear that the marriage was the second one (which was my own guess) - Manx legitimacy laws allowed at this period that children born within 2 years of marriage were legitimate.

There are guides to Manx family history on Manxnotebook