As Irelia I have a unit in base and I play Tideturner in base, can I choose another unit in base and make it ready with Irelia?
No, you cannot successfully use Tideturner to target a unit in your base while Tideturner is also in your base.
According to the card text retrieved for Tideturner:
"When you play me, you may choose a unit you control at another location. Move me to its location and it to my original location."
Why this fails:
- Targeting Restriction: The ability explicitly requires you to choose a unit at "another location." If you play Tideturner to your base, any other unit you control in that same base is not at "another location." Therefore, it is an illegal target for the ability.
- Triggered Ability: Since you cannot fulfill the targeting requirement ("a unit you control at another location"), you cannot select a target to place on the chain for the swap effect. If you attempt to activate it anyway (as the ability is optional/may), the targeting fails.
- Regarding Irelia: Because you cannot choose a target for the movement, the movement effect never resolves. Consequently, Irelia, Fervent will not be "chosen" or "readied" by Tideturner's ability, and her triggered ability ("When you choose or ready me...") will not trigger from this action.
In summary, Tideturner's ability cannot be used to swap with or ready a unit at the same location where Tideturner was played.