Irresistable spell is kinda broken.
An Irresistable Hideous Laughter is a 6th level spell that puts an enemy on the ground for 1 round per caster level, no save, no actions, close range (5th, for Bards).
Irressistable Dance is an 8th level touch-range spell that puts an enemy up and down (provoking, but not prone) for 1d4+1 rounds.
Which is better?
An Irresistable Charm Person has no real cognate.
An Irresistable Disguise Self is obscene.
An Irresistable Magic Aura lets you sell a masterwork sword as a +5 Vorpal Sword (no way to identify it, other than to dispel it first).
The above only take 5th or 6th level spell slots, normally, with the feat.
An Irresistable Magic Jar is more than worth the 9th level spell slot.
Likewise, an Irresistable Dominate Person, Baelful Polymorph, Feeblemind or Hold Monster makes a handy Win button in a 9th level spell slot.
That's all before the Incantrix ability to skip the level adjustment once per day. Irresistable Wierd? Irresistable Disjunction?
No. Don't. Your DM can munchkin better than you can. Your DM can try again and again if it doesn't work.