Hi Louisa -I am wondering if your Susanna's mother is the Mary McGonigle/McGonnell from Kilkeel, Co Down?
If so, Mary may be my 2x great grandfather Michael McGonnell's sister. He was born in 1837, from Moneydorragh, Kilkeel parish, and his father was Edward McGonnell (though appears as McGonigle in GV) and mother Rose Trainor. I am interested in finding siblings of Michael, and more about his parents.
Michael joined the Royal Navy in 1858 and left around 1861, ending up in New Zealand, where he now has many descendents.