Yup, in the first declension singular and plural forms simply get switched around in the genitive. Usually this simply involves slenderising the last consonant in the genitive singular and the nominative plural, occasionally the vowel changes as well (e.g. ‘fır’ from ‘fear’, ‘mıc’ from ‘mac’). There’s a reason for this too. The Proto-Celtic language had more pronounced case endings like Latin, which —depending on what vowel they contained— would ‘broaden’ or ‘slenderise’ the noun stem. The Irish forms ‘fear’ (nom. sg.), ‘fır’ (gen. sg.), ‘fır’ (nom. pl.), ‘fear’ (gen. pl.) are respective cognates of Latin ‘vır’, ‘vırí’, ‘vırí’, ‘vırórum’ (the slender vowels ‘ı’ and ‘e’ slenderised preceding consonants and sometimes triggered a vowel change, while other vowels caused the original ‘broad’ form to be retained). In Old Irish, almost all words lost their final syllable, so that all these endings were either largely reduced (mainly to -a and -e) or completely lost, leaving behind only broad and slender consonants (and occasional vowel changes in the stem) at the end of nouns to mark the cases. That’s why the distribution of forms does not always seem to make much sense.
The same dropping of the final syllable of most words in Old Irish is also why Irish has unpredictable initial mutations: in Proto-Celtic these mutations were caused by a letter at the end of a preceding word, now lost, that interacted in a predictable way with the following consonant (the ‘h’ and ‘n-’ sometimes added before vowels are remnants of these consonants).