It functioned in "one-to-all" identification mode, searching a large database of some million enrolled frequent travellers to see if anyone matched the presenting iris.
Thus IRIS exploited the great robustness of iris recognition against making False Matches, since (unlike weak biometrics such as face recognition) it could survive the vast number of opportunities to make False Matches when searching a large database, instead of only needing to test just a single asserted identity.
IRIS relied on biometric technology to authenticate identity and was part of the e-borders initiative of the UK Government.
The main factor behind the decision to decommission IRIS was the cost of staffing the enrolment offices in airport terminals, whereas face recognition only required submitting a Passport-type photograph acquired in a photo-booth without the need for personal presentation to airport enrolment staff.
[1] Enrolled passengers could enter the UK through a special automated immigration control barrier incorporating an iris recognition camera.