Hi mate and WELCOME TO THE FORUM :wave:
The only way your going to know what has triggered the air bag warning light (and re set the light) is to hook the car up to diagnostics equipment such as Multi ECU Scan or Fiat Examiner.
The airbag failure light being illuminated IS an MOT failure unfortunately.
Its important to note, that whilst the light is illuminated, your airbag may NOT trigger in the event of an accident, OR may trigger unexpectedly (rare). Therefore its important to rectify the issue ASAP.
Hope that helps,
Alan