My dad came to Sydney from Scotland in 1921 and mum in 1922, and were married that year in Leichhardt. Dad had pure Irish blood and mum Scottish.
I looked again at the other Isaac's and their parents and they all look legitimate.
Since we cannot find your Isaac's birth or christening, I looked to see if he was one of the other children. He shows born Laxey, Kirk Lonan on 1851 census , and YOB from 1812 to 1815 on Census and burial records.
John and Ruth had three boys born on the IOM
John 1803 German
William 1809 Lonan
James 1815 German.
William only one born Lonan.
John appears to be witness at Isaac's wedding, and James is shown as buried 3/6/1816, so that leaves William,
There are only 4 burials on Brian Lawson's list of a William Muncaster and none are the son of John and Ruth:
16/1/1834 Kirk German aged 2
10/11/1847 Kirk Braddan aged 4
27/11/1851 Kirk Braddan aged 75
29/12/1864 Kirk Malew aged 50
Looked for marriages - only 2 :
WM ( a Widower) married Hannah Miller 1833, Braddan, he had married Mary Collins at st James church Whitehaven ,England. Mary buried 25/2/1830 Braddan aged 50.
WM married Eliza Kinvig 17/5/1836 in Malew. From the 1851 census he is shown POB Lancaster, England , which is not quite correct, he was ch. 5/8/1810 St James , Whitehaven and is the son of William Muncaster and Mary Collins.( They have at least 10 children and most of them end up in Australia and New Zealand. He is the one who died 1864 aged 50 - 47 on 1861 census. His dad was the one who was buried in 1851.
So what happened to Isaac's brother William ? Did William become Isaac? Do any posters know if this is one of those English/Gaelic crosses like Ewen and Hugh ?
Can any poster pull my theory apart ?