A friend of mine, with whom I am teaching Old Irish, asked for some diagnostic features of Middle Irish so I thought I would put it here to help them and anyone else who may want to know.
There are many different features which mark the change from Old to Middle Irish. Much of this is discussed in much more detail in Kim McCone’s “A First Old Irish Grammar and Reader” pp. 173—217. I will give a small précis of this here. For a more full account for verbs, see Kim McCone “The Early Irish Verb” pp. 163—240. A full analysis by Liam Breatnach is detailed in “Stair na Gaelige” pp. 221—333. The main problem with Breatnach’s analysis is that it is entirely in Modern Irish so unless you know that language first, or you enjoy typing and trust Google Translate, it will be inaccessible to you.