> do·rónad < do-ro-gníd "has been made", augmented 3rd sg.
> preterite passive of do·gní.
Duh. Thanks for fixing that. Also -- to help fix it in
my mind -- I'll note that "do·rónad" must be the ancestor
of the Modern Irish past autonomous "(do) rinneadh".
The meaning, then, is that Ollam Fótla instituted the
Feast of Tara.