What happens after my application is approved - is there a ceremony or oath?

Approval is not the final step on its own. You are invited to attend a citizenship ceremony, and it is at that ceremony that you make the required declaration of fidelity to the Irish nation and loyalty to the State.

You become an Irish citizen on the date of that declaration, after which you can apply for an Irish passport. Until you have attended the ceremony and made the declaration, you are not yet a citizen, so do not finalise travel or passport plans before that point.