How does Irelia, Fervent chain with On Guard and Abandoned Hall?

Irelia, Fervent is a triggered ability, not a passive. When you cast On Guard and target Irelia, Fervent triggers and goes on the chain. After On Guard resolves, Abandoned Hall triggers for each spell resolved, and Fervent triggers again each time a spell resolves. You can add new items to the chain at any time, and the controller of the top chain item has priority first.