Ok of the couple Samuel Dunn b. 1854/3ish and Margaret McCreadie let me start with the Samual Dunn side.
From their marriage certificate I found his parents names were Michael Dunn and Margaret Dunn Nee Airdrie.
I have found a marriage in Ireland for one Michael Dunn and Margaret Airdry.
7 Sept 1848, Ballycopeland Presbyterian church, County Down.
Michael Dunn ~ Address: Donaghadee, Occ: Shoemaker, Full Age, Widower, Father : James Dunn, Shoemaker.
Margaret Airdry ~ Address: Donaghadee, Full Age, Spinster, Father: Adam Airdry, Farmer.
Samuel Dunn's father is listed as a gardner on Samuel's marriage cert but that's the only discrepancy.
This is as far as I've got on this line, I am stuck searching for records but finding nada. I looked at one that I thought might have been Michael Dunn's first marriage but no cigar. I was hoping that perhaps if there were any MI's of them in Rankinston it might give us some additional clues.
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On the Margaret McCreadie side.
I have her parents as James McCreadie and Agnes McQuater from her marriage certificate.
I also located her birth certificate as she was born in that magical year of 1855 it has a lot of info. It shows her parents were married in Portpatrick in 1854.
Their marriage record however is unfortunately just before civil reg so it's just a simple parish record that states they were both residing in that parish and got married on10th October 1854.
One thing I have noted is that James McCreadie must have been married before as on the 1861 census they have a 12 year old Sarah McCreadie living with them and listed as a daughter, this would place her birth before their marriage, plus Margaret McCreadie's birth cert states she was Agnes McQuaters first child.