Quick update.... Play Irelia Mid... At this point it doesn't seem like diana is going to get fixed anytime soon and I can probably list a bunch of things that make irelia feel like diana but better.
Mark passive: You get resets off of these marks from both champions; except irelia's does on hit (good with triforce), gives life back, fuels her passive, and her dash is on a basic ability and not her ult.
Dash: Both have dashes and both are reset by marks; except irelia's is on hit (mentioned), Heals (mentioned), fuels her passive (mentioned), and can be reset by killing the unit with the dash. Diana, works with mark (one mark until your next q), fuels passive, small channel (can use other abilities and items while dashing)
Defensive ability: This one is debatable. A refreshed shield is a life saver and so is damage reduction. But what makes irelia's so good is that you mitigate much more (potentially) and you don't need to hit someone to get the reduction. Diana get's an initial shield but the refreshed shield has to explode all orbs on an enemy unit.
CC: This one is no contest. Diana pull is something that people have been complaining about for a long time. Does no damage, pull isn't major and is fairly high on the cooldown. Irelia's CC, though it is a skill shot, it is greatly rewarded when it hits. You get a mark, Stack on your passive, Stun, Ranged and good for engage. Also, though it is a difficult ability, it has a learning curve and it is what makes irelia so hard to play.
Passive: I've mention passive though this post multiple times. Diana's passive (for those who don't know, you should probably not be on a diana forum) gives attack speed when you use an ability ( does not have to hit) and every 3 Auto Attacks allow you to cleave for 80% of your ap in magic damage, and gives you mana back. To be honest you can argue that their passive fit their kits extremely good but the one thing that breaks it for me is that diana has to keep casting abilities to keep using her passive. Once you proc your passive for irelia, it stays on while fighting large monsters and champions, helping you save your abilities for when you need them.
Overall Irelia just stay relevant the longer the game goes on and the longer she fights, which is something I would like in diana, but it is not possible for diana.
Btw you can build zhon on irelia so that was the only thing diana had going for her. sad face.
.