Thank you all so much.
Thank you also Maggsie for taking the time to lay all that out - do you have the baptism info for Mary too, just to complete the six?
Yes, I've been trying to piece something together from death records this morning, as dathai has suggested. I suppose I'm looking for another couple called Patrick and Elizabeth Duffy, as I don't think all these children here can be theirs, even though they said they had six.
While Mary could conceivably (sorry about that!) have been born 7 months after George, allowing for late registration, there is no way Sarah could have been born in May 1867 and Ellen in August 1867.
A long time ago I searched for a marriage between Patrick and Elizabeth on the Roots site and came up with only one in that area in 1855, too early probably for my research. But it's also true that a lot of Monaghan records are not on that site. In view of that, it's possible that there were other children born to P and E before the 1864 cutoff.