If a move into an open battlefield with Irresistible Faefolk and use her ability to pull in Fiora Peerless, does she get the bonus from her ability?

Yes, Fiora, Peerless gets her bonus.

When Faefolk moves to the open battlefield, you applied Contested and are the attacker (FAQ #11891, Rule 459.2.b.1). Faefolk's trigger resolves, pulling Fiora in, and she gains the Defender designation when the Combat Showdown begins. At that exact moment the battlefield holds only Faefolk and Fiora — one on one — so her trigger fires and doubles her Might (3 → 6) for that combat. Fiora's "one on one" condition is checked only at the moment she's designated (FAQ #1102, #11502), and here it's satisfied.