I would just call them trousers but maybe thats just me
Heaving a pair of trousers leads into a trap, translation only accepts bríste and doesn't accept péire bríste
This has more to do with English than Irish. In English, a pair of trousers is one thing, and this semantic object is encompassed by bríste in Irish. péire bríste would literally mean a 'pair' (i.e. two) trousers.
We have a problem with single nouns that end in -s, like trousers, pants, and scissors. I almost never hear "a scissors" anymore, even though I did regularly as a child.
I thought "tá bríste uirthi" would work, but it doesn't accept it