Revisiting this topic in light of recent replies on the other Nelson topic.
I do think that David Nelson (c 1805) married Margaret Woods in 1840 this would tie up the sons Robert (b 1843) and John (b 1846). I think his son David married Margaret Johnstone in 1849 ( Age around 25) and believe Jane Isabella (b 1863) to be his daughter.
David Nelson married Jane Agnes Dobbin with who he had several children. In 1840 he married Margaret Woods & they had at least 4 children. Margaret must have died after a few years and
in 1849 David married Margaret Johnston by whom he had at least 5 more children.
Davis Nelson had a son David who is listed in the 1825 & 1840 Visitation lists for First Garvagh Presbyterian Church. Normally it would have been difficult to trace him after those date if he didn't appear in local records or in Scotland like so many of the family. However, David went to America, married there (twice) and had children. One of his sons, Thomas Nelson, must have returned to Ireland to visit the Gortnamoyagh family because he married a local girl in 1884 and returned to America with her.
Added- although the 1849 marriage lists David Nelson as bachelor other information indicates he is the same one who married Jane Agnes Dobbin and Margaret Woods.