bríste is singular, brístí is plural. It doesn't matter whether you prefer to use "(a pair of) trousers" or "(a pair of) pants" in English. Tá brístí aige means "He has (pairs of) trousers/pants".
"pants" is an English word, so it doesn't refer to anything in Irish, but "pants" is often used to refer to underwear (fobhríste) in Ireland when speaking in English. Men's trousers are not generally referred to as "pants" in Ireland, though "pants" is used in "short pants", though just "shorts" would be far more common.
I'm not sure why you think that
Duolingo is translating "bríste" and "brístí" as "trousers" and "pants" respectively
Most of the exercises that use "pants" happen to be translated as brístí, but that's just indicative of the ambiguity of the English words.
I suppose I phrased that poorly. So far, I've noticed that when it's asking me to translate from English, it will say "the pants" when the answer it wants is "na brístí" and "the trousers" when it wants "an bríste." From what you've said, though, I think I might just be reading too much into that. So this may have been a stupid question to begin with, lol!