You can still find them there (the files are still on the server):
membrfaylz/anlondubh/The Séadna notes are the files
CaibidealX.pdf. I would also recommend looking at
Sliabh na mBan bhFionn (the SliabhnamBan.pdf file), which is the whole text by Peadar Ua Laoghaire and An Lon Dubh’s notes to it, and the notes are pretty beginner-level friendly. The text is quite short, so easier to get through the whole thing. The language, on one hand, is pretty archaic (story passed orally at least from 18th century without much change), on the other hand fairly simple (contrary to Séadna, it wasn’t written to show the whole richness of spoken Irish ;-)).
Also, there is a pretty faithful English translation of Séadna available on archive.org:
Shiana, helpful if you get stuck.
And then there is a whole subject
Study Group: Séadna (GM) on the Forum, where you’ll also find discussions of the language used in the book (and links to the notes).