I've seen that sometimes it is "na cait buí" and sometimes "na cait bhuí". I see it is lenition but why? Is it about those cats or the next word? Maybe that was explainen somewhere but I don't remember. Could somebody please explain me?
It should be cait bhuí. When you have a plural noun, if it ends in a slender consonant, the next work should be lenited (if possible).