So, I've made quite a few breakthroughs on this subject in the past couple of years. I apologise for not responding to all of your messages; RootsChat has not been notifying me of replies like it used to.
I discovered that David Tweedie had made a Poor Law application. I ordered a copy and there were a few enlightening facts. First of all, he was indeed born in County Down. He still gives his parents names as John Tweedie and Martha Pendleton. Still no insight on their birth or death dates, all I know is they were both deceased by the time he got married in 1876.
The most enlightening details were the descriptions of his siblings still residing in Ireland. They were described as follows;
Alexander, (married?), Carter, Belfast
John, (married?), Labourer, Lurgan
Thomas, (married?), Labourer, Moyallon
Robert, (married?), Labourer, Belfast
Eliza, single, Moyallon
He had not seen or heard from them for twenty years by the time of the application; 22nd Oct 1905.
Based on these details, I started searching through Irish records, and came to these conclusions;
Alexander Tweedy married Jane McCormick on September 29, 1859 in Banbridge, County Down. He was working as a Carman at the time. Four children have been found for the couple;
Jane Tweedy, born July 6, 1864 in Banbridge, Down
James Tweedie, born December 5, 1867 in Antrim
Martha Ann Tweedie, born January 15, 1870 in Antrim
Alexander Tweedie, born May 21, 1874 in Belfast, Antrim
John Tweedy married Margaret Kelly on November 6, 1857 in Magheralin, County Down. One child for the couple has been found;
David Tweedie, born December 27, 1865 in Lurgan, County Armagh
Thomas Tweedy was first married to Anne Willis on January 17, 1858 in Tullylish, County Down. He remarried (listed as widowed on marriage record) to Margaret Preshor on April 23, 1872 in Gilford, County Down.
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It's been previously suggested that Martha Pendleton was actually Pentland, and that a record exists for the marriage of John Tweedy & Martha Pentland in Donaghcloney, Down on February 5, 1823. However, I have been unable to locate the record.
I have baptism records for the following Tweedy's born in Lisnafiffy near Banbridge in County Down, all the children of a John & Martha Tweedie (no maiden surname supplied); Thomas in 1832, John in 1835, Alexander in 1837 and David in 1843. It seems very likely that these children are all siblings and match up with what we know about the family. Perhaps Robert and Eliza were born between 1823 and 1832?
One snag; there also exists a William Tweedy, born in 1830 in Lisnafiffy, whose parents are John and Martha. But in this record, Martha's maiden name is supplied as Patterson. This could be a mistranscription, but there is no way of knowing for sure.
What do you all think?