As a Darius player, I know that an Irelia who has both her passive and Conqueror stacked is untouchable. The wave is hers to do whatever she wants with, and I’ve gotta either sit offscreen or try to dodge her E. However, the instant her passive wears off, I know I smash her easily, and I will look for an all-in. Even if she manages to stack up while fighting me, it will be too late - because I will also have my stacks, and she will be at half HP.
Some things you can do as Irelia in this matchup... Just try to last-hit while your passive is stacked, whether you have the wave frozen or slow pushing, and keep an enemy caster minion at low HP. If Darius walks up, drop your E, Q to the caster, E stun him, Q to him, then just right click him to death. Save your Q after the reset for when he tries to land his Q. Save your W for when he tries to pull you. If he tries to Q while you’re in your W and he doesn’t have ult, tank it before recasting W. If he does have ult, recast immediately and Q inside his range - you don’t want him getting stacks. Congratulations, you just killed Darius.
If Darius catches you while your stacks are down and you have no minions to dash away to, save your E until after the slow from his E and W have worn off. Then you can stun him and walk away. Note that you cannot do this if he has Ghost and Mercs - if he does, you’re pretty much screwed.
But if he catches you while you’re fed and have your ult, then you just full combo him and kill him.