English doesn't have any other way to differentiate between one pair or trousers or multiple pairs of trousers - is "trousers" singular or plural?
Irish doesn't have this problem - bríste is unambiguously singular, brístí is unambiguously plural. Requiring "a pair of trousers" vs "pairs of trousers" simply highlights the important difference between bríste and brístí.
Orm/ort/oraibh etc all translate to "on me/you/he/she" and so on. They are prepositional pronouns.
"Tá bríste orm." - In this sentence you are literally saying "I have trousers on me." "Tá bríste ort." - "You have trousers on you"
Orm is a combination/shortened version of "ar mé" which is "on me."
Orm - ar mé Ort - ar tú Air - ar é
and so on..
It would be grammatically incorrect to say "Tá bríste ar mé" but maybe seeing it explained in this way may help.
Here is a table of prepositional pronouns.
I hope this helped!
Beir bua agus beannacht.