Did a bit of digging in 1st Garvagh records last night. Unfortunately Nelsons were not a family I extracted from the original records and am relying on information posted on Bann Valley site for the Visitation records. Data is full of all sorts of horrible errors so it all needs to be checked, but here goes...
1840 Visitation- Gortnamoyagh- David Nelson, Margaret, Joseph, David, Eliza, Catherin, Peggy Jane.
This list makes sense (i.e. Joseph b.c1821, Eliza c1829, Catherin c1831, Peggy Jane c1831). David's wife (I presume it is his wife) listed as Margaret. David Nelson supposed to be married to Jane Agnes Dobbins. Perhaps she died and David married again- more in a bit.
Next two households are listed as Dabbin (original needs to be checked). Possibly David Nelson came to Gortnamoyagh at the time of his marriage?
1796 Visitation not Nelsons at Gortnamoyagh but there are Nelsons at Tamneymore (not far away).
Now, there are 2 marriages in 1st Garvagh (these I have from the original):
David Nelson m.1840 Margaret Woods and
David Nelson m.1849 Margaret Johnston.
Also in 1840 at Gortnamoyagh are James Johnson, Kitty, Eliza, Jenny, James, Wallace, Kitty, Mary Ann, William, Margaret. Joseph & Jane (Johnston) Nelson had a son Wallace, so the Jenny listed in 1840 could be Jane Johnston.
I also copies from the receords several Nelson marriages (all with father named David so not easy to sort out):
Ellen m.1860 John Roxborough
John m.1873 Margaret A. Hutchinson
Mary Anne m.1876 Alex. Chambers
George m.1876 Fanny Anne Dunlop (this is in Moneydig Presbyterian Church)
Margaret m.1880 William McConnell (Aghadowey Presbyterian Church)
William m.1883 Martha Jane Ritchie.
Maybe one, or both, of you will be able to sort some of these Nelsons out better.