It's an interesting question, and I had to go research the answer in order to figure it out. What I think you are referring to is this from section of GnaG relating to the use of lenition for adjectives: "3. After a noun in the weak plural that ends in a slender consonant: na fir mhóra = the big men"
However, "*brístí" is a strong plural, since it is only a change in the suffix at the end of the word, as described in the GnaG section describing the Nominative plural. Therefore, the rule does not apply to this word.
Thanks for making me look up a new rule that I didn't know before. That was fun. :-)
As Mediterranean pointed out, the fact that brístí doesn't end in a consonant, broad or slender, is a rather more obvious reason why that rule doesn't apply.
brístí is only a "strong plural" because it used to be bristidhe - taken at face value, brístí isn't any "stronger" than bróga.
There are several types of declension for adjectives, but appending an -a for plurals is done for many of them. Some of the alternative declension types include crua → crua, leisciúil → leisciúla, breá → breátha, maith → maithe, tapaidh → tapaí, te → teo, and those that become syncopated.
There are two entries as follows
tapa1, m. (gs. ~). Quickness, readiness, speed; activity, vigour. ~ a bheith ionat, to be quick, ready, active. ~ a dhéanamh, to act fast; to hurry up. Is maith an ~ a rinne tú, you were quick off the mark. Bheith ar do thapa, to be ready, alert. Ní raibh mé ar mo thapa, I was taken unawares. Teacht ar ~, to come to the ready. Níl mórán ~ fágtha ann, there isn't much energy left in him. De thapa na huaire, by chance.
tapa2, a3. Quick, ready, active. Tá sé ~ as a lámha, he is quick with his hands. ~ ar a chosa, fast on his feet. Gníomh ~, quick, sudden, act. Déan go ~ é, do it quickly. Éirigh go ~, get up quick. Ag siúl go ~, walking fast. (Var: ~idh a1)
But I do not find any variant spelling. What is wrong ?