In a teamfight, it's generally best to aim Transcendent Blades to hit as many champions as possible to keep you alive, rather than directly at a single target for extra damage, as this will give you the best sustain. With proper positioning, both these effects can be achieved without having to compromise. For example, shift around to fire the blades forward into the bulk of the enemy team while also hitting the main target.
If coordinated well, it also makes an effective tool for escaping ganks or failed team fights at low health. By clicking the edge of your screen and firing off a few blades, you can heal from the enemy champions chasing you to buy time to escape.
Transcendent Blades can give you a "stepping stone". By using it to lower health of ranged minions, you can dash continuously when using Bladesurge on them.
Even though Irelia has a lot of sustain, she also has a lot of burst damage when her opponent has more health than her at level 6. Opening with Equilibrium Strike, activating Hiten Style and Transcendent Blades immediately, and Bladesurge as the opponent flees and gains some distance (so you can land another autoattack or even more with Phage. This combo is capable of dealing a lot of damage. Combined with Ignite and Sheen, it can even kill somebody from full health in a few seconds.
Irelia also can burst at level 2 (though you can only do that with a level advantage, or if your enemy has wasted an ability). Use your Bladesurge onto a low health minion near your enemy, then follow with aEquilibrium Strike, attack him and then Bladesurge after landing an auto attack or if he runs away. By using this trick, you can burn 2 of the enemy champion's health potions.
Phage adds onto chasing potential with Equilibrium Strike's CC, and additional AD/health is always essential on a melee champion. You should buy it first (after a Sapphire Crystal if you have mana problem).
Irelia is a champion that relies on attack speed. A Frozen Heart is a good counter to her AS.
Try to disengage when you see her Hiten Style is activating. It only has a 6s duration and the true damage and on hit heal allows her to trade. Try to harass her back when it is on cooldown.
Buying a Doran's Shield in the first trip back is a great way to counter Irelia if you are having trouble vs her, since most of her damage come from basic autoattacks but note that Hiten Style 's true damage can not be reduced by Doran's Shield.
After that, by using the same logic, you can buy Ninja Tabi or stack health (Giant's Belt is good since it has many upgraded paths).
Irelia's Equilibrium Strike makes her trading deceptively strong. Keep your current % of health higher than her or you will lose lane.
As a top lane champion that relies on reaching an enemy and sticking to them to apply her on hit true damage, picking a champion that can outdo her constant damage once she enters melee range like Xin Zhao or Udyr or disengage like Renekton or Jayce is always a good idea.
Ask for early ganks, shut her down early. Snowball champions like Riven are nightmares for Irelia to deal with when they are ahead. Though Irelia's power will allow her to turn the lane around if she gets enough levels. Riven, for example, cannot burst Irelia after enough levels have passed, causing Irelia to win lane.
It is recommended not to fight Irelia in minion waves. Her Transcendent Blades allows her to heal a lot of damage if it deals damage to both you and minions.
Try to counter her healing with healing reduction from Ignite. It can be quite powerful in early game.