Following up almost five years later, I've pieced together the following with the help of genetics and Irish and American records:
Farrell DONOHUE and Julia REYNOLDS do seem to have lived in the Townland of Annagh (Electoral Division of Drumgort, Civil Parish of Killoe) - specifically on a farm about a mile southeast of Loch Annagh and in between Corrinagh and Cloonback - at least according to Griffith’s Valuation. Here’s where it is on a modern map:
https://maps.app.goo.gl/mi5Wd7ZWL4gR7kbA6
I'm not sure if the buildings shown are original, but they also appear on the Griffith’s map.
Their sons James DONOHUE, born c1826, Michael DONOHUE born c1827-36, Bryan DONOHUE, born c1831, and John DONOHUE, born c1847, all went to New York in 1840s, 50s, and 70s and married there.
At least two of Farrell and Julia's children stayed in Longford: Patrick DONOHUE, born c.1840, and Mary DONOHUE, born c.1846.
Patrick DONOHUE - of "Annave" son of Farrell DONOHOE - married Catherine REILLY in 1872, and then settled in Smear - Catherine's home Townland.
Mary DONOHUE - of "Annave" daughter of Farrell DONOGHUE - married Eugene/Owen SHERIDAN in 1870 and raised several children in Aghakine/Ahakine/Aughakine. She died 13 Jun 1918 in Aghakine.
Farrell died 25 Nov 1879 at Annagh - and the death registration says he was married, but I haven't found a death record for Julia either before or after 1879. It's possible she emigrated. The informant was Edward DONOHUE, who died a bachelor at "Annave" in 1891 - apparently at age 70 - so he might've been their son, or Farrell's brother.
Farrell DONOHUE and Julia REYNOLDS might also have had other children, but I haven't yet made a connection via DNA - I do, however, have a number of DNA matches who descend from Bryan, John, Patrick, and Mary.
DNA also links the descendants of Farrell and Julia with the family of Gerald/Garrett REYNOLDS, c1830-1917, and Julia DONOHUE, who married in 1855, and lived in Annagh - however, I haven't yet figured out if Gerald/Garrett was the brother of *my* Julia REYNOLDS DONOHUE or if Julia DONOHUE was the daughter of Farrell and Julia.
Anyway - pardon the rambling - and if anyone has anything to add or suggestion, please do!