PIR is an acronym for the Public Interest Registry, a not-for-profit corporation responsible for managing the .org top-level domain.
PIR is an acronym for Public Information Research, a non-profit corporation that maintains Namebase, an online database of citations to influential people
PIR is an acronym for Private information retrieval, a protocol that allows a user to retrieve an item from a server in possession of a database without revealing which item she is retrieving.
PIR is Polyisocyanurate, a plastic typically produced as a foam for thermal insulation
Parent(s) In Room, signifying to people in chat rooms or instant message conversations that someone's parents are in the room and to watch what is said.
PIR is an acronym for the Ontario Ministry of Public Infrastructure Renewal
PIR is also an acronym for Post-Implementation Review