Farming is very important with Irelia. You schould never push too hard so that u can farm near your tower. After you get Trinity Force play much more agressive! But also its important to be warded that u dont get ganked. The first kill is the win top. So dont go for kills and play save if needed.

It is possible to play Irelia with Ghost, Ignite, Exhaust, Flash, Heal and Jungle Heal.
I think, that Flash isnt good at 5v5 because the enemys have it too. Heal is good but not so effective than ignite. With ignite you can get a kill after death or escape and earn the kill.
So i think the combo Ghost,Ignite/Exhaust are the best because so you can escape easy and get kills or defend the own ad carry/kill the enemy ad carry.

In team fights you must focus the enemy carry and save the own. Jump in at the enemy group and activate Randuins. Then focus their carry and slow if possible with your e spell. Also u shouldnt get more kills than assists, feed the carry of yout team!

One of Irelias jobs is it, to ward the Baron and toplane. But place the Wards right!
dont place the wards in missle of the bush, so you can see more. And dont place the ward in the last corner of the baron. Ward in front of it and so can the midlaner see ganks and eskape better.

Irelia is very good for ranked games. Its my favorite champ. But Irelia loose against some champs like Morde, AP Sion and often against Wukong. So buy first Armor/Magic regist and than u win!