That's fascinating, jonw65. Once again, you make an intervention that changes the whole picture!
Now that I know there was such a person, I can see that the witness is Bristow Johnson. Surely there can't have been a lot of people with that name.
I had not heard of the Henry Johnson letters. From the bit you cite from a letter of Henry's wife, Henry was a nephew of Alexander Mackey, so his mother was Mary Mackey who married William Johnson. You don't say how Bristow Johnson figures in the letters. But the Dublin marriage certificate doesn't say where the groom's father lives or lived, and there was a William Johnson, grocer, on Main Street Antrim in 1852. It looks plausible, pending refutation from the letters, that Bristow Johnson is a brother of Henry.
That means that if Mary McLorinan's father is the McLorinan who married Mary Mackey and fathered the Rev. Thomas McLorinan, Bristow Johnson was her cousin; their mothers were sisters. And that surely makes it likely that she, and by extension her probable sister Martha, belong in that family: sisters of the Rev. Thomas, daughters of Mary Mackey. That gives a nice explanation of why Bristow Johnson is a witness.
I still have a problem about the profession of the father-in-law of John McDowell, in whose presence Henry McLorinan, farmer, the person I thought was John's father-in-law, father of John's wife Elizabeth, died at Castle Street in 1875. Can that Henry McL be the same person as the father of Martha McLorinan, shopkeeper? And what about there still being a Henry McLorinan, grocer, on High Street in 1880?
I'm tempted to think the father-in-law of John McDowell junior, Samuel Johnson, grocer, is another brother. Perhaps he took over his father's grocery.
Thanks for all that, sam.b. But those McLorinans are surely Roman Catholic, whereas the Henry McL I'm interested in at least married a Methodist, and had at least one of his daughters married in the Church of Ireland. I don't think the McLorinans you have introduced are related. Of course if you go back far enough everyone is related to everyone else, but I think any connection in this case is quite distant. I've seen it suggested that the RC McLorinans in Co Antrim were Irish long before the Plantation, whereas the Protestant McLorinans may have been Scottish imports.