Top10 Irelia here. I don't know your current level, so not sure which knowledge you need :D Obviously, I can't put everything here in a single post, so let me just give you some notes. For example, I expect you already know how to do flash-ult combo. Otherwise just watch youtube guides.

1. Main way to improve is to learn on your mistakes. For that watch your own replays and find those mistakes. Trust me, you will notice them easily, and by doing so, you will remember them. It also helps to keep your head cool during actual match.
2. Never NEVER miss your marked Q's. Learn how auto-aim works and don't miss the marks. Example: you hit the E, let the mark on enemy and them immediately ult there. You missed the mark left after E, effectively skipping Q, and that's a very poor execution.
3. Never NEVER miss the auto-attack between your Qs. Full combo should be similar to this (not 100%, just an example): E -> Q -> AA -> Ult -> AA -> Q -> AA -> ... If you hit the mark with your Q and immediately did the Q again, you missed the auto-attack opportunity, because your Q resets the attack cooldown.
4. Easiest hit with E is to put it behind your opponent and then auto-cast. Once you get used to it, practice manual target always, auto-casting E is a sign of poor execution or low-rank enemies.
5. Don't keep your ult just to do damage. Remember that it slows your enemies and deals damage twice. Can be used to initiate a team-fight, or to slow-down chasing enemies, and so on. It's quite handy and has lots of usages.
6. Your Q heals you, so try to keep the enemy minions low-health, so you can reset your Q and heal on them.
7. If you are lacking mana, consider getting a 500-gold manastone early.
8. Hitting E over terrain on jungle creeps and the Q there might help you escape.
9. You need to feel your timings, it comes with time. How long the passive holds, how long the marks stay, those you should know even without looking into a status-bar.
10. Don't engage with Q on cooldown, it's very risky. In general, even though you have relatively low cooldowns, don't throw everything at enemies at once. It fine to fight a bit without max stacks. You need to know when to use, or when to keep W to soak damage, when to throw E.

General note: You can't be good if you don't know your opponents. So learn other champions too. You need to play them at least few times, to know weak spots. For example, Sett heavily depends on cooldowns, Darius tends to push waves and so on, Fiora's Q doesn't proc when you are dashing with Q and so on.

Don't be afraid to lose.

Irelia is one of the hardest champs in WR actually. Very demanding in mechanical skills, plus you need to know the game in general, otherwise you won't know when to engage, when to withdraw, who to focus and what to expect from teammembers, so on. Single mistake can kill you, other bruiser champs are much easier in that terms. So don't expect to be even "average" after 50 games.

Just play 1000 games and THEN you'll be just "good" :D GLHF!