There is an easy way to make Irelia's hybrid damage fair

Give her the Kayn (shadow assassin) treatment.

Passive: IONIAN FERVOR: Irelia's basic attacks deal 4 − 17 (based on level) (+ 4% bonus AD) Ability power bonus magic damage per stack, up to 16 − 68 (based on level) (+ 16% bonus AD). At maximum stacks, Irelia gains 30% / 40% / 50% bonus attack speed and deals an additional 32 − 136 (based on level) (+ 32% bonus AD) magic damage against Hybrid resistances icon shields.

Passive: IONIAN FERVOR: Irelia's basic attacks deal 5% / 10% / 15% / 20% of post-mitigation damage dealt against enemy units as bonus magic damage per stack. At maximum stacks, Irelia gains 30% / 40% / 50% bonus attack speed and deals an additional 40% of post-mitigation damage dealt against enemy champions as bonus magic damage.

Irelia deals too much hybrid damage to effectively itemize against. This makes armor her clear stat to use against her. Removed flat damage. Bad idea. Tweak numbers as needed.

BLADESURGE: HEAL: 12 / 14 / 16 / 18 / 20% AD Irelia needs to pay for sustain now.

Done. Her insane damage and her waveclear were both just hit by virtue of being mitigatable by one stat. She needs to buy lifesteal or take ravenous hunter and armor pen if she wants to reach her previous levels of damage/sustain, which would make her squishier.