From the little that I know about those matters, the work of R. Thurneysen has not yet been superseded for the oldest layers of Irish.

His Handbuch is still in print in English, entitled A Grammar of old Irish. I used to have it as a paper copy, which was quite expensive, but I don't remember how precisely it corresponds with this German edition.

I'm sure the specialists in Irish among us have had this one for a while, but for the rest of us, this might be fun.
I never succeeded in working completely through it. It has a reputation for being relatively intense and deservedly so.

With a few texts in the second part.

I'll be happy to post Strokes and Strachan's Thesaurus palaeo-hibernicus if there is any demand for it.