I gather you already know the answer (and just wrote your comment as an exercise to the reader), but if anyone is baffled by this and cannot find the reason, here it goes:
the feminine nouns in plural don’t cause lenite their attributive adjectives, so one long year is bliadhna fhada, but multiple long years are bliadhnaichean fada.
To confuse the learner more, some masculine nouns – those that form their plural by slenderization (and not with the ending -an) – do lenite the following adjectives in plural: na cait mhòra the big cats, na balaich bheaga the small boys, etc. But if they form their plural by additional ending, then there’s no lenition: na h-eileanan beaga the small islands.
Yes, thanks. I'm collecting exceptions and rules that are "quasi-arbitrary" - they wouldn't be rules otherwise ;-) - to post under example sentences that give me pause. The use of adjectives as predicative modifiers and/or attributes; the feminine plural form that blocks lenition; the masculine plural that creates lenition ... etc. There's been lots of good moderators and Gaelic teachers on these forums in the past few weeks. Very helpful, thanks!
Thank you for your explanation silmeth . In addition, would I be right in thinking that the plural of the definite article does not cause lenition in the following noun in the way that the some singular definite articles can. As in a' bhliadhna and na bliadhnaichean and is this true for all plural definite articles?