I think that the best way to nerf Irelia is to make her stun harder to hit.

This could work by either shrinking the stun's hitbox a bit or by making the blades travel a little slower. If Irelia misses her stun, she loses out on up to 3 potential stacks of her passive, which is a pretty decent steriod, and her mobility drops a bit. Immobile champions might also have a chance to play the lane if she can't as reliably chunk them out.