This is my guide to Irelia on patch 8.12. I played her to level 7 in 3 days after the rework and have a >70% winrate in ranked on her.

Press the Attack: This is better for most matchups in which you want to get early kills and snowball. The damage increase also helps your team, but it doesn't stack, so if you know that an ally takes it, it probably isn't the best idea.
Conqueror: This is better to beat tanks once they have some resistance. It also should be the better option if an ally has PTA.
Electrocute: You'll probably only want to use this on the burst builds I presented. You can try to cheese a squishy toplaner with your normal build too tho, for example against teemo.
Dark Harvest: A very cheesy option for the AD-Assassin build. Will make your early worse tho.
Resolve options: Bone plating + Chrysalis/Conditioning. Conditionioning if you want to build very tanky.
Domination options (as secondary): Sudden Impact (overall damage) + Eyeball Collection (snowball) / Zombie Wards or Ghost Poro (for vision) // Ravenous Hunter (sustain) / Ultimate Hunter (more all in fights)

You will max your spells in the order in which you use them the most with, of course, the Ultimate taking priority.
Normal build: R>Q>E>W
Ap build: R>E>W>Q // R>W>E>Q W gives more survivability, while E gives more openings for all ins. The skillpoints can be spread between both aswell.
Ad Assassin: R>Q>E>W // R>Q>W>E

You can skill E on Lvl 1 in case of an invade. In that case go Q on Lvl 2.