Hello, I know you don't care about this part so I'm going to make it short.
My ingame name is Qvkata De El Ge on the EUNE server, I am an Irelia main with currently 760k champion points (yes, I've played her in quite some games).
I first started playing this game in preseason 6 where I tried Irelia for the first time and fell in love with the champion.
The things I like the most about her is how she can win every match up with ease, is an assassin with good sustain and survivability. So you're a tank assassin - how cool is that?
Anyway, lets get started. In the upcoming parts of the guide I will explain to you everything that I have learned about Irelia and some helpful tips overall.
Enjoy.

+Oneshots
+Insane mobility near enemy minions and in teamfights
+Has high outplay potential
+One of the best duelists
+Insane one item spike

-Doesn't scale very well into the late game.
-Her mobility is heavily situational unless maxed Q with lots of cdr
-Horrible frontliner alone
-Success in teamfights mostly depends on flanking
-Heavily reliant on her ulti while dueling

-Q, Irelia's defining spell. You dash to an enemy (minion or champion) and do damage based on your ad + applies on hit effects (W+ lets say wit's end). That sums it up, right? Wrong. Each time you kill a target your Q cooldown resets enabling you to Q to another target. This is the ability that makes or breaks irelia. If you're good with managing this spell, you're most likely a great Irelia player. TAKE NOTE THAT CASTER MINIONS DIE INSTANTLY FROM W+Q WITH TRINITY FORCE. ALSO THIS WILL BE YOUR UNDER TURRET FARMING TOOL, ALWAYS USE IT IF YOU ARE SHUVED AND FORCED TO FARM UNDER YOUR TURRET. Also stray from heavy trades if this spell is on cooldown.

-W, This spell gives you passive on hit healing (meaning you can heal from damaging turrets) on active you do, depending on the level, true damage plus increases the on hit healing effect. You would want to max this spell out first 99.99% of the time, because the true damage is just stupid when you have a couple of points invested into it. Don't try to force trades while this spell is on cooldown.

-E, Pretty straight forward - if your enemy has higher % HP than you or you are both equal at hp (like 100% hp both) it will stun the target for a maximum of 2 seconds depending on level, if the target has a less % HP than you at the moment of cast, you slow him instead. Keep in mind that at max level the stun is 2 seconds which is more than enough to kill a squishy using your full combo. Also a trick you can do is you can wait for your enemy to hit you a couple of times if you are higher % HP than him, so you can stun him instead of the slow when he is lower hp than you.

-R, ah, your ultimate ability, the sheen stacking tool. Amazing spell which does wonders when combined with the rest of her kit. It fires 4 blades in a target location (skillshot) which do both magic and ad damage (which synchronises well with wit's end if you would decide to go for that item in your build). The blades will heal you for an amount depending on the damage the blades do and the ammount of targets that it hits - champion hits heal for a larger ammount (try to position yourself in such a way that you hit your enemy champion while also hitting his minion line for a maximum ammount of healing (trust me the healing combined with your turned on W is just stupid high). ALSO YOU CAN USE THIS SPELL TO LOWER THE HP ON MINIONS NEUTRALS ETC, SO YOU CAN KILL THEM WITH A SINGLE Q AFTER THAT(SO IT CAN RESET AND YOU CAN GO ON YOUR TARGET OR JUST RUN AWAY FROM A BAD SITUATION) (caster minions take 2 ulties so that your Q kills them (if you do not have sheen).ALSO THIS SPELL GENERATES SHEEN PROCS, SO TRY SYNCHRONISING IT WITH SHEEN'S COOLDOWN SO YOU CAN GET THE MAXIMUM AMOUNT OF SHEEN ENHANCED AUTO ATTACKS WHILE DUELING.

Irelia's early game is pretty bad, until you get at least sheen, or phage. I like to play passively till i get my powerspikes (I feel most comfortable at around level 5). In hard matchups its all about surviving to that sweet sweet trinity power spike. So dont be scared to get pushed in at the start, your Q does wonders in farming. Since irelia is pretty squishy, and doesnt really have a reliable disengage except flash, it's better if you play a bit more passive.

In this section i would like to talk about the do's and do not's in the laning phase.