I've found two John McMillans born about the right time in Wigtownshire because I now believe that is where he was born.

Just a thought...

First family father William and mother Agnes - William McMillan and Agnes both born in Ireland!

Second family father James and mother Elizabeth - James McMillan and Elizabeth both born in Ireland.

John named his first son James and his first daughter Elizabeth!

I think John's parents were Irish too now.