Extracted from here:
The prevailing custom in the Diaspora to relegate the Birkat Kohanim to the major holidays. After Birkat Kohanim, the congregation silently recites the short Adir bamarom prayer.
When Birkat Kohanim is not performed, the chazzan recites the short prayer, in lieu of Birkat Kohanim, "Our G d and G d of our fathers, bless us with the threefold blessing written in the Torah by Moses Your servant, and pronounced by Aaron and his sons the Kohanim… [and the prayer concludes with the text of the Priestly Birkat Blessing]."
Is there a source for saying the Adir bamarom prayer after the in lieu prayer?