I don’t understand how this is the plural of breitheamh. How does this paradigm work?
Breitheamh is a first declension noun, and there are many paradigms within the first declension nouns — some of them have weak plurals, and some of them have strong plurals. Another noun with a similar strong plural formation to breitheamh is ollamh, with its plural ollúna.
...I’ma need some historic background to make heads or tails of this...
Seems to be a slight bug in the mobile app... It told me I have a too in my answer, but I selected the words from the options offered, and my answer matched exactly. Still marked it correct. Weird.