Farming and patience. I do not play Irelia all the time but she has always been a bit of a burst assassin and a bruiser. Put her against tanker bruisers it has always been about attrition then striking when they are low.
Try focusing on farming and if Darius comes in to harass you can stun/slow aa him a bit and back off. Wait till you have healed off creeps a bit. Rinse repeat. By the time he has mogs you should have enough farm to get some armor which cripples Darius or enough damage to push the lane relatively fast and hank those squishes or clear some jungle.