its not just Airbus, its Honeywell, so this may ref other AC as well...
An ADIRS consists of up to three fault tolerant ADIRUs, an associated Control and Display Unit (CDU) and remotely mounted Air Data Modules (ADMs).
Each ADIRU comprises an Air Data Reference (ADR) and an Inertial Reference (IR) component.
Now GPIRS may be different, as there are some IR units that are directly coupled with GPS to update directly, instead of by the Pilot/GPS.
The FMS will use GPS until the GPS degrades past the filter limits, then will use the IRS/INS. There is a drift value associated with the IRS, typically 5nm/hour, without update.