When moving an Irresistible Faefolk to an uncontested battlefield, am I the attacker after moving an enemy unit to the battlefield Faefolk moved into?

Yes, you are the attacker.

According to RiftJudge FAQ #9581, #9828, and Rule 459.2.b.1, the determination of attacker and defender designations is based on which player applied the "Contested" status to the battlefield first.

Sequence of Events

1. Move: You move Irresistible Faefolk to an unoccupied (uncontrolled) battlefield. This applies the "Contested" status to that battlefield. Because you are the player who applied this status, you gain the Attacker designation.
2. Trigger: The "When I move to a battlefield..." ability of Irresistible Faefolk triggers. You place this ability on the chain.
3. Resolution: When the ability resolves, you choose an enemy unit and move it to that same battlefield.
4. Combat Transition: Because units from opposing players are now present at the contested battlefield, the showdown transitions into a Combat Showdown.

Even though your ability moved the enemy unit, you remain the attacker and the enemy unit becomes the defender because you were the one who initiated the combat by applying the "Contested" status to the battlefield first.