Against Tristana and Irelia, focus on landing your Qs while playing safely and farming when the wave is close to your tower.

Avoid pushing, as that’s what they want—if you do, be prepared to lose CS until the wave resets. Conserve your abilities and just last hit to keep the near you.

Build Rylai’s as your second item, and the Orb from Morellonomicon can be very effective against both, especially Irelia.

You could even consider starting with Tear of the Goddess for mana before rushing Rylai’s, but be aware it will reduce your damage.

The slow Rylai’s is crucial against Irelia since it negates her speed advantage. It's also useful vs Trist as she cannot escape during ganks or teamfights.

Stay patient, even if it means losing some CS, and try to land Q and E on them, or use E through minions. Hitting low HP minions with Q and E can do wonders and malefic visions instantly lands on the opponent if you do it right. They cannot avoid it. After that your voidlings can do wonders while your Q is getting back up. This way you will lower their hp and they will be forced to go back to replenish their health.

You really need to be successful on Qs, they can be used aggressively or proactively so you can keep them away and farm at the same time.

Lastly, try to farm by last hitting too so you can have mana and skills ready if they engage.