This is the family in the 1911 census:
The 1901 census for that townland appears to be missing. Here’s Matthew’s marriage to Mary Roulstone. (He married the girl next door. Many farmers did).
Matthew’s father was also named Matthew. With Mathew senior and Alexander having farms very close together they are likely related.
Mathew senior appears to have been married to Rebecca Kilgore:
That Mathew’s father was another Mathew:
Alexander died in 1889 aged 90:
Here’s the death of Ellen Long in 1877. I suspect she was Alexander’s wife:
This 1861 marriage indicates an Allen Long had likely farmed in Carricknamanna. He had presumably died before 1857:
Here’s 1 of Alexander’s daughters marriage:
Tradition was to marry in the bride's church. The above marriage was in Donaghmore Presbyterian so that suggests it’s the place to look for baptism records etc.
One of Matthew’s daughters marriage:
(There are probably more marriages to be found but I stopped looking at this point).
The 1911 family were Presbyterian. That suggests they are probably descendants of Scots who settled in that area in the 1600s. There are no Longs in the Muster rolls for Donegal c 1630 so they probably arrived after that. There were Longs then in Tyrone & Londonderry so they may have moved the short distance into Donegal post 1630.
The tithe applotment records for Donaghmore c 1830 appear to be lost.
There are 2 Presbyterian churches in Donaghmore. Carnone has records from 1834 onwards and Donaghmore from 1835. They are not on-line anywhere as far as I am aware but there are copies in PRONI (the public record office) in Belfast. They are free to view there. Getting back any earlier will be pretty well impossible due to the general lack of records for the 1600s and 1700s.