Maybe not a myth but a misnomer. You are claiming it to be an Irish thing.fuck knows mate.
You said the Irish granny bit was a myth. It isn't.
It appears with several countries, not just an Ireland only situation.
It was sarcastically called the " granny rule" by the British press when Ireland were playing a lot of UK born players in the Jack Charlton era.
I'll give you an example with a Man City connection.
Brahim Diaz now plays International football for Morocco as his grandmother was born there.