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.
"Ar" means "on", but instead of "ar mé", you have "orm", (on me) and instead of "ar tú" you have "ort" (on you).
Literally, this sentence means "Trousers are on you", which is the Irish way of saying "You're wearing trousers."
"Le do thoil" translates literally to "with your will", because there's no direct word for "please" as Gaeilge.
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.