The OB's great-great grandfather was born in England to English parents but emigrated to Australia where he married a girl born in Ireland to Irish parents. So the grandfather was 100% Australian by birth but 50% English and 50% Irish by blood. He came to England and married a girl born in Liverpool to parents who were both born in Ireland. So grandma was 100% English by birth (though as she was born in Liverpool she was really Scouse) and 100% Irish by blood. The OB's father, therefore, born in Hackney, London, was 100% English by birth but 25% Australian and 75% Irish by blood. That means the Old Bat is 3/8ths Irish by blood, 1/8th Australian by blood and 1/2 English. But like I say, she prefers to think of herself as wholly English if not Sussex.
(I should have posted this last autumn but never did.)