Sengoídelc: Old Irish for Beginners by David Stifter

It's difficult. You get some good background, and a great amount of Old Irish example sentences, a lot from real sources, but it piles on the information quickly and the transcription for phonetic stuff is difficult to read (using non-standard Greek letters to represent phonemes, instead of IPA symbols). There's a lot of reconstruction examples of Proto-Celtic through to Old Irish, which is nice, but maybe unnecessary for true beginners.

I'm sure if you have the patience to get through the density of the first few chapters, it'll be a great resource, but otherwise, it's an awkward read. (and it's about as massive as Wheelock's Latin).

This textbook does dick you around.