Nancy Stenson's grammar workbooks ("Beginning Irish" and "Intermediate Irish") provide the clearest explanations of the structure of the language that I have seen anywhere, in print or online. I first read both volumes like novels and am now working my way through them more carefully, doing the exercises as I go. On the initial read through there were so many "ahah" moments that I almost quit being surprised. This is coming from someone who has worked painstakingly through "Learning Irish" -- twice! -- and who has combed through several other Irish grammars and general courses.
The exercises are proving to be well thought out and quite useful for fixing the concepts in my mind. Overall a very smartly put together set. I wish she would come back and write a general course for beginners. I would love to have it for the classes I am teaching.
As mentioned by another reviewer, this series does not include audio recordings. There is a very good "Spelling and Pronunciation" chapter in the first volume, but to hear the language you need to include other resources. As explained in the introduction of each volume: "This is not meant to be a self-contained course or reference grammar, both of which are readily available elsewhere. Rather, it is intended to accompany and supplement whatever course materials an individual or class is using."
In my opinion, this two-volume set has met and exceeded these intentions.