My irongrip says its undispellable when I cast it, however the Watcher dispelled it with force of will:

our acidic bite MANGLES the Watcher!
Your defilement MANGLES the Watcher!
The Watcher's searing light devastates you!
The Watcher's searing light devastates you!
The Watcher narrows its eyes and glares in your direction.
Your hand starts shaking, but you do not drop the double-bladed axe of Trel'aran
.
The Watcher's force of will hits you.
The magic of your iron grip of azazel spell unravels.
The Watcher has a few scratches. <90%-99%>

Incidentally, this is also the (game balance) reason force of will does a little damage in that case.