If a player rolls two or more 1s when rolling power dice to cast a spell, this is a very bad thing. Note that miscast spell does not normally succeed, regardless of the actual casting result rolled by the Wizard.
The Wizard, and by extension the player, is about to find out what kind of terrible consequences occur when foolish creatures dabble in things best left alone. The player must roll 2D6 and consult the.
Obviously the chance of rolling a miscast or irresistible force is increased by rolling more dice. Such is the nature of magical power! If a spell is miscast and is cast with irresistible force at the same time (eg, from five dice the player rolls two 1s and two 6s) then the spell counts as a miscast. Miscasts take precedence in this case.
With miscast and irresistible force, it is the actual dice scores that are considered, irrespective of bonuses and before any re-rolls from magic items are taken. Unless otherwise specified, a re-roll cannot cause a spell to be cast with irresistible force, nor can it prevent a miscast.