If I play an Irresistible Faefolk to an occupied battlefield and move an enemy unit from base to that battlefield, am I then the attacker still or do I become the defender?

When you move Irresistible Faefolk to a battlefield that is already occupied by an opponent's unit, the battlefield is already contested (or becomes contested by the arrival of your unit).

According to Rule 459.2.b.1 and the established RiftJudge FAQs regarding this specific card, the determination of attacker and defender designations is based on which player applied the "Contested" status to the battlefield first.

1. If the battlefield was already contested by your opponent: Your opponent is already the attacker. By moving your unit (and subsequently pulling another enemy unit) to that battlefield, you do not change the attacker/defender designations; your opponent remains the attacker and you are the defender.
2. If the battlefield was NOT already contested: By moving your unit to an occupied battlefield, you are the one who applies the "Contested" status. In this scenario, you are the attacker and the units already present (and the one you moved) are the defenders.

Summary

• If your opponent was already the attacker because they contested that battlefield first, moving an Irresistible Faefolk there does not change their status. You remain the defender.
• If you are the one applying the "Contested" status (e.g., you move to an uncontested battlefield, or you are the first player to move into a battlefield that was previously empty), you are the attacker.

This interpretation is supported by RiftJudge FAQ #9423 and #10794, which clarify that if you move into a battlefield already contested by your opponent, they remain the attacker regardless of your unit's movement ability.