The description of this skill is not correct. Irresistible Blow always does extra damage, but then if the attack is blocked, the target is knocked down and suffers additional damage.
Since this skill is so good....is there any particular reason why I'd want to take another skill like Mighty Blow?? The only "disadvantage" with this skill is just its dependence on energy. But give the fact that Mighty Blow takes so bloody long to charge up anyway...why???