Julie, I have a 3x great aunt Susannah Irvine/Irwin who married an Abraham Alcorn, farmer at Drumacarney, near Limavady. I don't know her dob but guess it to be about 1800.
I have a family tree you sent me in 2008 that is tantalizing in its similarities, but we just could not make the information fit at that time. Susannah and Abraham had children Eliza, John and James. I am guessing that Abraham is the first name; the notes only say "Susannah married Abraham" but as that followed "Peggy Ann married Jos. Allcorn" I thought it possible Abraham might be an Allcorn as well. But "Jane married Richie" suggests not all the husbands' first names were known.
William was born about 1803 and their first known child (Oliver) was born in 1832, so a sister of Betty born ca 1800 would be a good fit.
Here are Betty's siblings again. Note that it says she was one of nine children and there seem to be at least ten total.
"Wm. Kerr married Elizabeth Irvine (my grandmother) one of nine children. Wm, Alexander, Samuel, Ellen, Peggy Ann, Susannah, Jane {married Ritchie in America/ Michigan near Kalamazoo/had a dau. Jane} and one John (?) who went to England. Peggy Ann married Jos. Allcorn. Susannah married Abraham [Allcorn/Alcorn?] { Eliza/John/James}."
If you read my previous post about how Samuel Hazlett might be a cousin to Alexander Kerr, you may see that I think the best chance is if I can find a Hazlett-Irvine connection. The other piece of information I have is that Alexander and his brother John seem to have been accompanied by a Leslie Irvine / Irwine (age 18) when they emigrated. All three were listed as laborers. I believe Alexander inflated his age by two years; he also was listed as 18, and John as 19. In the 1841 Londonderry census abstracts I find a possible match for Leslie as child of Samuel and Margaret Irwin. Again, I can't quite fit Leslie into your 2008 tree!
Name: Leslie Irwine
Arrival Date: 13 May 1856
Age: 18 Years
Estimated Birth Year: abt 1838
Port of Departure: Londonderry, Ireland
Ship Name: Superior
Port of Arrival: Philadelphia, Pennsylvania
Microfilm Roll Number: M425_79