Irelia/Strategy

Jungle

 * will allow you to heal yourself and kill jungle spawns quickly.
 * can be helpful (if you consistently last-hit with it) or hurtful (if you don't)
 * Once you have high Attack Damage, combined with you can kill off the weaker spawns in a second. This will also assist you in covering unguarded towers from large minion waves allowing you to return to the jungle much faster.

Laning

 * gives a very useful low-cost disable allowing you to heavily slow the enemy while doing a large amount of damage.
 * even heals you while attacking structures like turrets.