The oaths are kept with the will and other papers in the probate file in PRONI. In your case the PRONI website says they hold no papers. That implies the original was destroyed in 1922, together with the oaths and other accompanying papers. Since you have a copy of the will, I would assume it must have came from some other source (eg the deceased’s solicitors).
The executors were usually close friends of the deceased. They would find the beneficiaries addresses from speaking to next of kin or by going through the deceased’s papers. But I have never seen beneficiaries addresses recorded within a probate file other than when it was in the will itself. (Solicitors often discourage you from putting a private beneficiary’s address in a will, as people move around and if the address has changed by the time the person inherits, it can cause complications. They suggest if there is any need for an address, you put it in a separate letter which accompanies the will but is not part of it. I have never seen that type of information in a probate file at PRONI).