(as generous * (as) blackbird)
As generous as a raven.
This simile is quoted in O’Mulconry’s Glossary (§310, Archiv i 248) in the entry for "díbech" (niggardly, stingy), where it is used to support a fanciful etymology of that word:
"Aliter dibech .i. ni in fiach, ar in fiach dia fagba sasad congair a cele cuige, unde dicitur: is felithir duben: non sic in dibech."
= Another explanation: díbech, that is "un-raven", because when he finds food, the raven calls his companion (to come) to him, from which is it said: "he is as generous as a black bird"; not thus the niggardly one.
The idea of the generous raven is also encountered in Shakespeare's "Titus Andronicus" (ii.3):
"Some say that ravens foster forlorn children,
The whilst their own birds famish in their nests."
My thanks to Neil McLeod and David Stifter for their assistance.