Some clarification ...
Charlotte Susan Magrath b26.9.1864 Dublin, buried 11.7.1953 Dublin was the daughter of Robert Charles Magrath & Cecilia Magrath (previously Flynn nee Rich).
Susan Charlotte Magrath (but actually christened Susanna)b8.2.1862 Dublin & who migrated to Australia, having married Christopher St John French, was the daughter of William and Elizabeth Magrath.
When Charlotte Susan was born in 1864 the 2 familes were living at 72 and 78 Capel St Dublin. Robert Charles and William's fathers were called William, so could be they were brothers - but no proof of this last so far - has anyone else?