I agree with the majority of the comments here.
Irresistable Phantasmal Killer, Slay Living, Charm Person, Dominate Person, Suggestion, Blindness (as a 6th level spell no less), etc are too good for their level. In fact, they're probably too good for a non-epic game.
Irresistable Zone of Truth and Irresistable Detect Thoughts (both sixth level and therefore available far before Mind Blank which protects against them) would also be potential game-breakers if the game were more politically focussed.
Like many of the feats in the Kalamar book, Irresistable spell is overpowered and probably unsuitable for most campaigns.