"My pants are dirty."
Translation:Tá mo bhríste salach.
You don't simply lenite an adjective because the noun is lenited. Attributive adjectives agree with their nouns in case, number and gender, so an attributive adjective will be lenited if the noun is feminine - even if the noun itself isn't lenited (because it starts with a letter than can't be lenited) - níonn sé an léine shalach - "he washes the dirty shirt". But the possessive adjectives like mo don't affect case, number or gender, so they don't have any impact on the adjectives of nouns.
(Note that in this case, salach is a predicative adjective anyway, so agreement wouldn't occur even with a feminine noun - tá an léine shalach ar iarraidh - "the dirty shirt is missing", but tá an léine salach - "the shirt is dirty" - note the position of "dirty" in each of those sentences).