Yes — a plural noun that ends with a slender consonant lenites its following adjective, and the ending slender consonant is due to the last vowel being slender. A plural noun that ends with a slender vowel does not lenite its following adjective (at least, not for that reason).
"A plural noun that ends with a slender vowel does not lenite its following adjective (at least, not for that reason)." Hmm, that doesn't sound like what the grammar notes say. How do I report a problem with the grammar notes (under the Colors section)?
> If the noun ends with a slender consonant, the adjective is also lenited.
Then they explain that a slender consonant is one with a slender vowel next to it
It is because it ends in a slender consonant, yes (meaning the last vowel is a slender one)
There isn't "a second one". In Irish, attributive adjectives are modified to reflect the case, number or gender of the noun that they qualify. The particular modification that applies in this case is that the adjective is lenited.