Is there not a rule that consonants can suffer lenition after a masculine noun in the nominative/null if it ends slender?

I know the basic rules says this does not happen, but I have seen examples (none on me thought)

There a rule that adjective is lenited after a noun in nom. pl. when that noun ends in a slender consonant. The rule applies to both masculine and feminine nouns, but there is just a handfull of feminine nouns with slender consonant in nom. pl.

Examples: fir mhaithe, lachain bhána

Go rabh maith agat/Gorama'at