Yeah, if you were gonna use “sona,” it’d technically be unlenited here along with in “lá breithe/breithlá…” Off the top of my head, I’d guess that the permanently lenited form “shona” has probably spread under the influence of things like “Nollaig shona…” &c.
Compare also lenited “mhaith” in some dialects after other words that are normally masculine, e.g., “eolas mhaith,” “cleachtadh mhaith,” in Connemara Irish, again maybe having come about by analogy with phrases related in meaning but where the nouns are feminine, as “aithne mhaith,” “taithí mhaith”…
There are yet other examples of lenited adjectives after theoretically masculine nouns, too, or indeed of adjectives that seem to be more or less permanently lenited when used with certain meanings, or when used adverbially. Cf., e.g., “diabhal thiománta,” “níl a fhios agam shaolta,” “ní raibh deoraí shaolta riamh ann,” &c.…
