When irresistible force occurs, first resolve the effect of the spell that the Wizard was attempting to cast. As the spell has been cast with irresistible force, the casting player can at least enjoy the effect of his spell before something almost indescribably bad happens to the Wizard and everyone nearby.
Once the effect of the spell has been resolved, the Wizard now needs to roll 2D6 on theto see what happens to him.
You'll notice that many of the results on the Miscast table call for damage (in the shape of automatic hits) to be caused upon the Wizard or nearby models. Though we've yet to talk about how damage is worked out, you'll find all the information you need in step 4 of the Shooting phase.
Our Wizard from the previous example has presumably wrought significant destruction on the enemy with his irresistibly cast The Dwellers Below. However, it's now time to pay the piper. As the spell was cast with irresistible force it was also miscast, and the Wizard must roll on the Miscast table to discover his fate. He rolls 2D6 on the Miscast table and scores a 7, resulting in a Detonation! result. As a result, all models in base contact with the Wizard suffer a Strength 10 hit and D6 dice are lost from the power pool. Painful, but it could have been much worse for the Wizard!
Note that some magic items and special rules talk of automatically triggering a miscast or irresistible force. This is the only occasion on which one can be had without the other.
So, for example, a magic item that caused a Wizard to miscast automatically would force that Wizard to roll on the Miscast table, but the spell would not be cast with irresistible force.
Similarly, an item that automatically generates irresistible force would not require a roll on the Miscast table.