If it was bright for some time it would not normally be an iridium satellite,they are just normal satellites that flair at a certain point then fade back to normal brightness.
The brightness of the iridium flair is only at the best a few seconds, so for the satellite to be brighter than the ISS for any length would be an anomaly?
As for cutting out sharply it has just gone into the earths shadow.
If you follow satellites in binoculars they can be seen long after you can see them with the unaided eye.
How long was it bright for,and was the brightness constant untill the fade out.
There are thousands of satellites out there,but none brighter than the ISS which can reach -8.5-9.0 magnitude.
Cheers
