Thanks Josey I spotted this too. It's a coincidence because Richard (Sarah's husband) is also from Cork.
My heart sank when I saw there were two Sarahs on the marriage record because I wasn't sure which it was, but since they later have a John Leehan/Sheen boarding with them it is possible.
Starts to get a bit mind boggling at this stage!

ETA: I think that must be right - Mary's surname is hard to make out on this census but I think it must be Mary Sheen, sister in law. The age/DOB matches from the older census (1871) when they lived with parents John and Elizabeth Sheen, and the newer census (1881) where Mary is living with the married Richard Mahoney and Sarah Mahoney (née Taylor).
But it would be interesting to know what happened to the original Father!
Thanks again, as always everyone is really helpful
