Hi Lilian,
My father's family were Dubliners, and my grandfather only came to Australia in or about 1919. His birth certificate records his mother's name as Isabella Anne Brabason. However, his older siblings' birth certificates record their mother's maiden surname as Isabella Anne Browne. His father, Henry Elliott Smith, was married to Isabella Anne Browne, not Brabason. She was one of at least two daughters of Stephen and Jane Browne (nee Bailey). Her sister was Martha. I don't know if it is relevant but there is a lot of Huguenot blood in the Smith and Elliott families, with their forebears moving to Ireland in the wake of the St Bartholomew's Day massacres.
I have spent some years trying to get to the bottom of this, and can not find any rational explanation, hence my belief that it will probably remain a mystery.
Thank you for your interest: knowing little of how these things worked in Ireland in the 19th century makes it a bit hard to find answers, especially from such a distance.