Hi Tara,
I have Mary O'Brien, daughter of John O'Brien and Susan, who married Myles McLoughlin in 1865 in the ProCathedral (my great-great-grandparents).
Known facts are that her sister Sarah, married a Richard Leon (they have been relatively easy to find documents for, though the discrepancy here is that the Latin version of her mother's name on the marriage in 1869, looks like Marianna). Also, that her brother James moved to Bolton and had a son called Fred and established a furniture shop (this also leads to various available documents).
I thought at first that the John who married Mary Anne Quinn in 1853 (also ProCathedral) would be a brother, but then came across another family where the father had married twice and one of the older children had listed his parents as father and the current wife at his marriage, so began to wonder).
From age at death, Sarah was born about 1848, so precedes that 1853 marriage and there were children at under 2 year intervals for quite a while after it, so difficult to see where she would fit in as a child of John and Mary Anne.
Facts about Michael are that he says his parents are John and Susan at marriage, but that earlier than that, there is a Michael living at John and Mary Anne's address who says he was born about 1832 in Kilkenny (the criminal record). Possibly also a Patrick, born in 1834, but I haven't yet traced the family back to Kilkenny and, if the business was as large as it seems from the later bankruptsy proceedings, than there could have been cousins involved too!
The more obvious story is that John married Susan and set up his business in Kilkenny and then, as it expanded, moved to Dublin. It could also have been a family business as one reference is to a James at one of the earlier addresses. Susan must have died between 1851 (when their son, also called James, was born) and 1853. I can't find a burial, but perhaps that was back in Kilkenny, or perhaps just not able to match to the right burial with so little info to distinguish her? John, left with several very young children, remarried quite soon and the started the second family. However, despite many different documents, I have no documentary evidence that the Michael who married in 1873 is the one with the criminal record in 1863. Attempts to find his burial, to determine his age, have failed so far.
With no idea of Susan's date, or place of birth, or maiden name, I have no way of finding out more about her directly!
I've also no evidence of John's age or place of birth and can't find his burial either.
If that Michael and Patrick with the criminal records, do belong to John and Susan then I will need to be looking back to Kilkenny, and there would be plenty of time for more children in the first family.
Unfortunately, with the surname O'Brien, sometimes found just as Brien, those names are really difficult to isolate.
Kate and Elizabeth O'Brien turn up as godparents, but only after a time when these could be Catherine, the first daughter of John and Mary Anne, and Elizabeth Reid, the wife of Michael, so possibly red herrings.
It's a strange brick wall as part of the problem is that I have so many documents but can't find the detail to tie the family together properly!