Can anyone else hear the difference between capall and capaill, or béar and béir? It sounds identical to me. I guess this question could apply to any weak plural.
Good question and I also have a lot of difficulty with this, so I usually check forvo when I've got lots of time. I went over to forvo to listen to both words in singular and plural form. For capall/capaill, they sound different, but they are pronounced by two different people, so it's hard to know whether they really sound different, or if it's just because two different people are pronouncing them.
For béar/béir, the same person pronounces both, and they do sound quite different. Rather than attempting to transcribe into something no one will understand, I'm going to put links to them below.