> Oidce tar Oidceanta = Lesson upon lesson
I suspect these are actually mistranscribed by the project. That looks more like it should be "Oiḋċe sa Coill" or "Oidhche sa Choill" without the ponc séimhithe, and in modern spelling "Oíche sa Choill" - "A Night in the Forest". Comparing the transcription of the first chapter with the source in the PDF they're missing a fada (an acute accent for non-Irish speakers) in "ná".
Similarly, I'd probably render the second one as "Night upon Nights".
Not a native speaker myself, just a former gaelscoil student who's done their best to undo the gaelscoilis tendencies. Probably closer to a "heritage speaker" in the linguistic sense in some aspects.
Sadly out of practice these days, since I've been living in Denmark nearly three years. It's strange to lose competency in a language that you spoke every day for about 13 years.
I hope the project can upload a full scan at some point. I'm a huge sci-fi fan, and there's definitely a dearth of Irish language books in that genre.
The line between folklore and mythology is fuzzy, but this definitely falls on the mythology side of the line.
The battle/fight with the Craidmi. troid is singular whereas war is plural.